Integration Guide

• Dependency
  • Gradle
  • Maven
• Overview
  • Jactl.eval()
  • Sharing Data with Scripts
  • Setting input/output for scripts
  • Jactl.compile()
  • Input/Output
  • Errors
• JactlContext
  • javaPackage(String pkg)
  • minScale(int scale)
  • environment(JactlEnv env)
  • debug(int level)
  • classAccessToGlobals(boolean accessAllowed)
  • Chaining Method Calls
• Compiling Classes
• Location of Jactl Scripts and Classes
• Jactl Execution Environment
  • getThreadContext()
  • scheduleEvent()
  • scheduleBlocking()
  • Example Implementation
  • .jactlrc File
• Checkpoints
  • saveCheckpoint()
  • deleteCheckpoint()
• Adding New Functions/Methods
  • Parameters
  • Default Values
  • Exception Handling
  • Suspending Execution
  • Integration with REPL and CommandLine Scripts
  • Async Parameters
  • Async Instance
  • Handling Resumption
• Example Application

This guide details how to integrate Jactl as a scripting language into a Java application and how to extend the language with additional methods/functions.

Dependency

To use Jactl you will need to add a dependency on the Jactl library.

Gradle

In the dependencies section of your build.gradle file:

implementation group: 'io.jactl', name: 'jactl', version: '2.0.0'

Maven

In the dependencies section of your pom.xml:

<dependency>
 <groupId>io.jactl</groupId>
 <artifactId>jactl</artifactId>
 <version>2.0.0</version>
</dependency>

Overview

Jactl.eval()

The simplest way to run a Jactl script is to use the io.jactl.Jactl class and the eval() method:

Object result = Jactl.eval("3 + 4");         // Will return 7

Since this has to compile the script each time, it is not the most efficient way to invoke Jactl scripts and, due to the way it waits synchronously for a result even if the script does an asynchronous operation, this call is not generally suitable if running in an event-loop based application. (The preferred way to run Jactl code is to use Jactl.compile() to compile into a JactlScript object that can then be invoked multiple times as discussed below).

Sharing Data with Scripts

To share data between the Java application and the script, it is possible to pass in a Map of global variables that the script can then access.

Map<String,Object> globals = new HashMap<>();
globals.put("x", 3);
globals.put("y", 4);
Object result = Jactl.eval("x += y", globals);
int    xval   = (int)globals.get("x");         // xval will be 7 

Jactl supports the following object types as values for the global variables:

• Boolean
• Byte
• Integer
• Long
• Double
• BigDecimal
• String
• List
• Map

For Maps, the keys must be Strings and for both Lists and Maps, the values in the List or Map should be one of the types listed above.

It is also allowed that the value of a variable is null which can be used as a way to create the variable when there is no initial value that makes sense for it.

Objects which are instances of a user defined Jactl class are also supported, so, if a previous script invocation has returned such a value, then this same value can be passed to another script invocation. Note that this requires that both invocations use the same JactlContext object (explained later):

JactlContext context = JactlContext.create().build();
Object x             = Jactl.eval("class X { int i; def f(n) { i * n } }; new X(2)", Utils.mapOf(), context);
Map<String,Object> globals = new HashMap<>();
globals.put("x", x);
Object result = (int)Jactl.eval("x.f(3)", globals, context);
assertEquals(6, result);

Setting input/output for scripts

If the script uses the nextLine() function to read lines from some input then you can specify a BufferedReader as for the script’s input. Similarly, you can also supply a PrintStream object that will be where any output from print or println is put.

For example:

Map<String,Object> globals = new HashMap<>();
globals.put("prefix", "DEBUG:");
BufferedReader input  = new BufferedReader(new InputStreamReader(System.in));
PrintStream    output = System.out;
Jactl.eval("stream(nextLine).each{ println \"$prefix: $it\n\" }", globals, input, output);

Jactl.compile()

The preferred way to run Jactl scripts is to compile them using Jactl.compile(). This returns a JactlScript object which can then be run as many times as needed.

JactlScript objects can be run using the runSync() or run() methods.

If a scripts perform an asynchronous or blocking operation (for example invoking sleep() or performing a database operation) then Jactl suspends the script and resumes it once the result is ready. This allows event-loop based applications to run Jactl scrips without worrying about blocking the event-loop thread that invokes a Jactl script. The runSync() method works like eval() in that it waits for the script to complete before returning the result to the caller. If invoking scripts from an event-loop thread of your application be aware that this might therefore block that thread if the script does something asynchronous. If the threading model of the application requires that the result of an asynchronous operation is processed on the same event-loop thread that invoked the operation (for example Vert.x based applications) then using runSync() will cause the event-loop thread to block forever if the script does something asynchronous since the thread waiting for the script to complete is also the thread that the result needs to be processed on before the script can return. This is why runSync() should only be used if the caller can guarantee that the script does not invoke an asynchronous function or if the caller is not running on an event-loop thread.

Map<String,Object> globals = new HashMap<>();
JactlScript script = Jactl.compileScript("3 + 4", globals);
Object      result = script.runSync(globals);          // result will be 7
assertEquals(7, result);

Again, you can use the Map of globals as a way of sharing data between the script and the application. The globals you pass in at compile time should contain all the global variables that the script will refer to. The values in the Map can all be null at this point since the compiler just uses the Map to decide if a variable exists or not.

The globals Map passed into the runSync() call (and the run() call) will be then be the one that the script uses at runtime. This can be a different Map each time but should, obviously, contain an entry for each variable passed in at compile time or you will get a runtime error when the script tries to access a global variable not present in the map passed in.

The run() method should be used in situations where you don’t want to block the current thread (for example, because you are already on an event-loop thread). It takes two arguments:

1. the globals Map for global variables, and
2. a completion (of type Consumer<Object>) that will be passed the script result when the script completes.

Note
If the script is entirely synchronous (doesn’t use any asynchronous functions like sleep()) then the completion will be invoked in the current thread before the call to run() returns to the caller.

Here is an example:

Map<String,Object> globals = new HashMap<>();
globals.put("x", null);
globals.put("y", null);
JactlScript script  = Jactl.compileScript("x + y", globals);

Map<String,Object> globalValues = new HashMap<>();
globalValues.put("x", 7);
globalValues.put("y", 3);
script.run(globalValues, result -> System.out.println("Result is " + result));

Input/Output

Both run() and runSync() have overloaded verisons that also accept a BufferedReader and PrintStream for the input/output of the script.

Errors

If an error is detected at compile time then an exception of type io.jactl.CompileError will be thrown.

If a runtime error occurs then an exception of type io.jactl.runtime.RuntimeError will be thrown.

A special subclass of RuntimeError called io.jactl.runtime.DieError will be thrown if the script invokes the die statement.

All these exception classes are subclasses of the top level io.jactl.JactlError.

Note
All of these classes are unchecked exceptions so be sure to catch them at the appropriate place in your code.

JactlContext

Although most of the examples shown so far haven’t used a JactlContext, in general, you will need to create an object of type io.jactl.JactlContext when evaluating or compiling scripts. The JactlContext allows you to set some options (including the execution environment - see below) and provides a way to compile sets of related Jactl scripts and Jactl classes. Jactl classes can only be referenced from scripts and classes that are compiled using the same JactlContext. For example, this provides a level of separation if you want to be able to run a multi-tenant application where each tenant has their own set of scripts and classes.

To create a JactlContext object you call the class static method create() followed by a number of fluent-style methods for setting various options and then finally invoke build() to return the instance.

So to get an instance with defaults for everything:

JactlContext context = JactlContext.create()
                                   .build();

Then to use the JactlContext you have built, pass it to the Jactl.eval() or Jactl.compileScript() methods:

JactlContext context = JactlContext.create()
                                   .build();

Map<String,Object> globals = new HashMap<>();
JactlScript script = Jactl.compileScript("13 * 17", globals, context);
script.run(globals, result -> System.out.println("Result: " + result));    // Output will be: "Result is 221"

Object result = Jactl.eval("13 * 17", globals, context);
assertEquals(221, result);

javaPackage(String pkg)

Jactl scripts are compiled into Java classes. They have a Jactl package which could be '' if unspecified or something like x.y.z if a package declaration exists at the top of the script.

In order not to clash with Java packages, the Jactl package hierarchy is placed inside a default Java package of io.jactl.pkg. Therefore, scripts without a Jactl package will end up in the package io.jactl.pkg and with a package declaration like x.y.z the package will be io.jactl.pkg.x.y.z.

If you would like to change what Java package the Jactl scripts and classes should reside under you can use the javaPackage() method when building your JactlContext:

JactlContext context = JactlContext.create()
                                   .javaPackage("io.jactl.pkg.abc")
                                   .build();

Note
If you are trying to keep different sets of Jactl scripts and classes separate from other sets of scripts and classes then it makes sense to put them in different package hierarchies to avoid any name clashes.

minScale(int scale)

By default, Jactl uses BigDecimal for numbers with decimal points (although double is also supported). Since BigDecimal supports arbitrary precision we need to decide, when doing division, how many decimal places to keep if the result has a non-terminating series of digits after the decimal place.

The default is to take the maximum precision of the two operands plus an additional 10 digits (the minScale value). You can change the default of 10 by using the minScale() method:

JactlContext context = JactlContext.create()
                                   .minScale(5)
                                   .build();

environment(JactlEnv env)

The JactlContext is also used to configure what runtime execution environment you are using. If no environment is specified, Jactl defaults to using an environment of type io.jactl.DefaultEnv which uses two java.util.concurrent.ExecutorService based thread pools: one for the event-loop threads and another one for the blocking threads.

If you are running in a different type of event-loop based application then you should create a JactlEnv class appropriate for your environment (see Jactl Execution Environment for more details), or use one provided (for example the io.jactl.vertx.JactlVertxEnv environment class from the jactl-vertx library).

Here is an example that specifies a JactlVertxEnv object as the execution environment:

Vertx vertx          = Vertx.vertx();
JactlVertxEnv env    = new JactlVertxEnv(vertx);

JactlContext context = JactlContext.create()
                                   .environment(env)
                                   .build();

debug(int level)

The debug() method allows you to enable debug output. It outputs the generated compiled JVM instructions for use when troubleshooting.

The supported values are:

• 0 — no output (the default value)
• 1 — output instructions
• 2 — output instructions with additional information about code locations

classAccessToGlobals(boolean accessAllowed)

This method controls whether access to the global variables is allowed from within a class (methods and field initialisers).

By default, classes are not allowed to access globals and access will result in a compile-time error. If for your application it makes sense for classes to have access to globals then you can invoke this method with true.

Chaining Method Calls

The methods for building a JactlContext can be chained in any order (apart from create() which must be first and build() which must come last). So to explicitly build a JactlContext with all the default values:

JactlContext context = JactlContext.create()
                                   .javaPackage("io.jactl.pkg")
                                   .environment(new io.jactl.DefaultEnv())
                                   .minScale(10)
                                   .classAccessToGlobals(false)
                                   .debug(0)
                                   .build();

// This is equivalent to:
JactlContext context = JactlContext.create().build()

Compiling Classes

As well as compiling Jactl scripts, you may also need to compile Jactl classes. As mentioned previously, Jactl classes, once compiled, can only be accessed directly by other Jactl scripts and Jactl classes that use the same JactlContext.

To compile a class use one of the Jactl.compileClass() methods:

JactlContext context = JactlContext.create().build();
Jactl.compileClass("class Multiplier { int n; def mult(x){ n * x } }", context);

Then to use the class, compile and run a script that refers to it using the same context:

JactlContext context = JactlContext.create().build();
Jactl.compileClass("class Multiplier { int n; def mult(x){ n * x } }", context);

Map<String,Object> globals = new HashMap<>();
JactlScript script  = Jactl.compileScript("def x = new Multiplier(13); x.mult(17)", globals, context);
script.run(globals, result -> System.out.println("Result: " + result));

Note
When compiling classes there is no way to pass in a set of global variables because Jactl classes have no access to global variables. Global variables are only accessible from Jactl scripts.

If the class has a Jactl package declaration and the script is in a different package, then the script should either import the class from the other package or use a fully qualified class name:

JactlContext context = JactlContext.create().build();
Jactl.compileClass("package a.b.c; class Multiplier { int n; def mult(x){ n * x } }", context);

Map<String,Object> globals = new HashMap<>();
JactlScript script  = Jactl.compileScript("package a.b.c; def x = new Multiplier(13); x.mult(17)", globals, context);
script.run(globals, result -> System.out.println("Result: " + result));

// Or import the class
script  = Jactl.compileScript("import a.b.c.Multiplier; def x = new Multiplier(13); x.mult(17)", globals, context);
script.run(globals, result -> System.out.println("Result: " + result));

// Or put script in same package
script  = Jactl.compileScript("package a.b.c; def x = new Multiplier(13); x.mult(17)", globals, context);
script.run(globals, result -> System.out.println("Result: " + result));

When compiling a class you can also specify the Jactl package name for the class explicitly:

JactlContext context = JactlContext.create().build();
Jactl.compileClass("class Multiplier { int n; def mult(x){ n * x } }", context, "a.b.c");

The Jactl package name passed in can be:

Value	Description
null	Error if package not specified in class declaration.
""	If package name not specified in class declaration then it will default to "" and be placed in root package.
any other value	If not specified then default to this value. If specified it must match this value or an error will occur.

There is also a similar Jactl.compileScript() method where you can pass in the Jactl package name for the script as well:

JactlContext context = JactlContext.create().build();
Map<String,Object> globals = new HashMap<>();
String pkgName = "a.b.c";
JactlScript script  = Jactl.compileScript("def x = new Multiplier(13); x.mult(17)", globals, context, pkgName);

The only reason for passing an explicit Jactl package when compiling a script is so to allow the script to access classes in the same package without having to explicitly qualify them with a package name or to import them.

Note
Like classes, scripts can specify a package themselves using the package directive and the rules about how package names apply are the same as for classes (see table above).

Location of Jactl Scripts and Classes

In the examples we have shown we have always used constant strings as the source code for our Jactl scripts and classes but this is unlikely to be the case in a proper application. In general, the scripts and classes will be read from files or from the database since the whole idea using Jactl to is to that Jactl scripts provide a way to customise the behaviour of an application.

See Example Application which gives an example application that reads Jactl scripts on demand from the file system in order to respond to incoming web service requests.

Jactl Execution Environment

In order for Jactl to support event-loop based applications with non-blocking execution of scripts, it needs to know how to schedule blocking and non-blocking operations.

An implementation of the io.jactl.JactlEnv interface must be provided that supplies a bridge from Jactl to the execution environment in which it is running.

The interface has the following methods that need to be implemented:

public interface JactlEnv {
  Object getThreadContext();
  void scheduleEvent(Object threadContext, Runnable event);
  void scheduleEvent(Object threadContext, Runnable event, long timeMs);
  void scheduleEvent(Runnable event, long timeMs);
  void scheduleBlocking(Runnable blocking);
}

There are also two other optional methods with default implementations (saveCheckpoint() and deleteCheckpoint()) that will be described below in the section on Checkpoints.

getThreadContext()

In order to support event-loop based applications where resumption of a long-running operation should occur on the original thread that spawned the long-running task, there is a method called getThreadContext() that should return a handle that identifies the current event-loop thread, and which, when passed to scheduleEvent() will allow the new event to be rescheduled on the same original event-loop thread.

If there is no such concept of rescheduling on the same thread then getThreadContext() is allowed to return null. If getThreadContext() is called from a non-event-loop thread then it should return null unless it makes sense to return a context for a specific event-loop thread (for example if there is some way for it to know that the currently running code is somehow related to a previous event on a specific event-loop thread).

scheduleEvent()

The scheduleEvent() methods are used to schedule non-blocking events onto an event-loop thread.

If passed a thread context that was returned from getThreadContext() then the event should be scheduled onto the same event-loop thread. If not passed a thread context then it is free to pick an event-loop to schedule the event on.

If passed a timeMs value then the event should be scheduled to run after the specified amount of time has expired.

scheduleBlocking()

This method is used to add execution of some blocking code to a queue for execution by a blocking thread once one becomes available.

Example Implementation

As an example of how to write a JactlEnv class have a look at the io.jactl.vertx.JactlVertxEnv class provided in the jactl-vertx project or see the default io.jactl.DefaultEnv implementation.

.jactlrc File

If your environment class has a no-arg constructor, and you want to be able to run your extension functions and methods (see below) in the Jactl REPL or in commandline scripts, then you can configure your .jactlrc file with the name of your environment class.

See .jactlrc File for more details.

Checkpoints

Jactl provides a checkpoint() function that allows scripts to checkpoint their current state with aim of then being able to restore this state elsewhere and continue execution from where the script left off in the event of an outage. Jactl provides the checkpoint state as a byte[] object, but it is up to the execution environment in which Jactl is embedded to provide a way to preserve this state and to then decide when it is appropriate to continue the execution state of the checkpoint on another instance (or after a restart on the same instance, for example).

The checkpoint state could be stored to a database, or replicated over a network, or written to the file system. It is up to the application to decide what is appropriate based on the overall architecture of the solution. If the scripts never invoke checkpoint() or there is no need to replicate or preserve script state then this feature can be ignored.

The JactlEnv implementation discussed above has two additional methods for dealing with checkpoints:

1. saveCheckpoint(), and
2. deleteCheckpoint()

If an application wants to be able to save checkpoint state then the application should provide an implementation for both of these methods, however, since these methods have default implementations that do nothing, it is not mandatory to provide implementations for these methods.

saveCheckpoint()

The signature for this method is:

  void saveCheckpoint(UUID             id,
                      int              checkpointId,
                      byte[]           checkpoint,
                      String           source,
                      int              offset,
                      Object           result,
                      Consumer<Object> resumer);

The parameters are:

Parameter	Description
id	UUID that uniquely identifies the running script instance.
checkpointId	If the script checkpoints multiple times this identifies which checkpoint we are up to. It is guaranteed to be an incrementing number with no gaps.
checkpoint	The actual checkpointed state of the script instance as a byte[] object.
source	The location in the source code where checkpoint() is called (for error reporting).
offset	Offset into source where checkpoint() is called (for error reporting).
result	The result that should be passed to the resumer once the checkpoint has been saved.
resumer	A callback that resumes execution of the script instance once the checkpoint has been saved.

All script executions are given a unique UUID to identify the executing instance regardless of what actual script is being executed. Over the lifetime of the application many script instances will be executed for many scripts and the UUID identifies each of these instances.

When a script instance invokes the checkpoint() function to checkpoint its state it keeps track of how many times it has been checkpointed before and the checkpointId parameter is an incrementing count that allows us to identify the checkpoint for that particular script instance. The combination of the id and the checkpointId uniquely identifies a given checkpoint state.

The call to saveCheckpoint() is invoked from an event-loop thread so implementations need to be careful not to block this thread while saving the checkpoint.

Once the checkpoint has been saved, the resumer object should be invoked, passing in the result or a RuntimeError object if an error has occurred. It is up to the implementation to make sure that this is done on an event-loop (non-blocking scheduler) thread.

A naive, incomplete, implementation of saveCheckpoint() (one that doesn’t scale) could look like this:

  static JactlEnv env;
    
  @Override
  public void saveCheckpoint(UUID             id,
                             int              checkpointId,
                             byte[]           checkpoint,
                             String           source,
                             int              offset,
                             Object           result,
                             Consumer<Object> resumer) {
    // If environement supports scheduling on a specific thread then remember current thread
    Object threadContext = env.getThreadContext();
    
    env.scheduleBlocking(() -> {
      Object retVal = result;
      try {
        FileOutputStream fileOutput = new FileOutputStream("/tmp/checkpoints.data");
        fileOutput.write(checkpoint);
        fileOutput.close();
      }
      catch (IOException e) {
        retVal = new RuntimeError("Error persisting checkpoint", source, offset, e);
      }
      Object finalRetVal = retVal;
      
      // Make sure resumption of script is done on event-loop thread (pass in threadContext if environment
      // supports scheduling onto a specific thread)
      env.scheduleEvent(threadContext, () -> resumer.accept(finalRetVal));
    });
  }

Note
Once a checkpoint has been saved it is safe to delete the previous checkpoint for that script instance. Alternatively, you can wait for the final deleteCheckpoint() call to know when to clean up the old checkpoints.

deleteCheckpoint()

In order to know when it is safe to clean up old checkpoints, at the end of a script instance execution the deleteCheckpoint() is invoked, passing in the id of the script instance and the last checkpoint id that was saved for that instance.

The deleteCheckpoint() call is done on an event-loop thread and, since there is no need to actually guarantee that the deletion has completed before returning, if your implementation does anything that will block then you should make sure to schedule the work on a separate blocking thread.

Adding New Functions/Methods

Since the idea of integrating Jactl into an application is to provide a way to customise behaviour, it is likely that the application would want to offer some additional global functions and methods that the Jactl scripts and classes can use.

For example, assume we want to provide a method on byte arrays for encoding into base64 and a method on Strings for decoding from base64:

> def x = [ 1, 2, 3, 4] as byte[]
> x.base64Encode()
AQIDBA==
> 'AQIDBA=='.base64Decode()
[1, 2, 3, 4]

In addition, to show how we can add new global functions, we will use the example function sendReceiveJson() that is provided by the jactl-vertx project. It is an example function for sending/receiving JSON messages over HTTP:

> sendReceiveJson(url:'http://localhost:52178/wordCount', request:[text:'here are some more words to be counted'])
[response:8, statusCode:200]

To make Jactl aware of new methods and global functions we use Jactl.method() (for methods) and Jactl.function() (for functions) and use fluent-style methods to build the method/function, before finally invoking .register() to register it with the Jactl.

For example to register the base64Decode() method the following should be invoked sometime during application initialisation (before any Jactl scripts/classes are compiled):

Jactl.method(JactlType.STRING)
     .name("base64Decode")
     .impl(Base64Functions.class, "base64Decode")
     .register();

The Jactl.method() method takes an argument, being the type of object on which the new method should exist. For base64Decode() the method exists only on String objects, so we use JactlType.STRING to specify the Jactl type for Strings.

The types that can be used here are:

Type	Description	Java Type
JactlType.ANY	Any object	Object
JactlType.STRING	Strings	String
JactlType.LIST	Lists	List
JactlType.MAP	Maps	Map
JactlType.ITERATOR	Anything that is iterable (List,Map,String,Number)	Object
JactlType.BOOLEAN	boolean objects	boolean
JactlType.BYTE	byte objects	byte
JactlType.INT	int objects	int
JactlType.LONG	long objects	long
JactlType.DOUBLE	double objects	double
JactlType.DECIMAL	BigDecimal objects	BigDecimal
JactlType.NUMBER	All numeric types	Number

Note
In Jactl, unlike Java, primitive types can have methods defined for them.

For the base64Encode() method we need to specify that the method applies to byte arrays, so we use the JactlType.arrayOf() method and pass in JactlType.BYTE to specify an array of bytes:

Jactl.method(JactlType.arrayOf(JactlType.BYTE))
     .name("base64Encode")
     .impl(Base64Functions.class, "base64Encode")
     .register();

For global functions, there is no type that owns the function, so we use Jactl.function() instead of Jactl.method() to construct the new function object and then use the same fluent methods to provide information about the function to Jactl:

Jactl.function()
     .name("sendReceiveJson")
     .param("url")
     .param("request")
     .impl(VertxFunctions.class, "sendReceiveJson")
     .register();

For both methods and global functions, the name() method must be specified as shown in the examples. This is the name that Jactl scripts will use to refer to the method or function.

The other mandatory method is impl() which tells Jactl the class and the name of the static method to invoke for the implementation of the method/function. In addition, if a third argument is passed to impl() it is the name of a public static field of type Object in the implementing class that the Jactl runtime can use to cache some data needed for the function/method. If no third argument is present it will default the name to the static method name appended with Data.

So this:

Jactl.method(JactlType.STRING)
     .name("base64Decode")
     .impl(Base64Functions.class, "base64Decode")
     .register();

is the same as this:

Jactl.method(JactlType.STRING)
     .name("base64Decode")
     .impl(Base64Functions.class, "base64Decode", "base64DecodeData")
     .register();

Note
Each function/method being registered needs to provide its own public static field of type Object. These fields cannot be shared between functions.

If there are no parameters to the function/method then this is all that is needed before invoking the register() method to register the function.

For methods, the first argument to the static implementing method will be the object on which the method is being invoked. For the base64Encode() method, the static implementation function should take a byte[] argument, and the base64Decode() static function should take a String argument. Since there are no parameters for these methods, the implementing class can be as simple as this:

  public class Base64Functions {
    public static String base64Encode(byte[] data) {
      return new String(Base64.getEncoder().encode(data));
    }
    public static Object base64EncodeData;

    public static byte[] base64Decode(String data) {
      return Base64.getDecoder().decode(data);
    }
    public static Object base64DecodeData;
  }

Parameters

Our example base64Encode() and base64Decode() methods did not have any parameters but the sendReceiveJson() global function takes two parameters: the URL to send to, and the request object to be sent as JSON. When registering a method or function, if there are parameters then they need to be declared using the .param() method. This allows Jactl to support named parameter passing. For example, we declared the url and request parameters for the sendReceiveJson() function like this:

Jactl.function()
     .name("sendReceiveJson")
     .param("url")
     .param("request")
     .impl(VertxFunctions.class, "sendReceiveJson")
     .register();

This allows us to invoke the function from Jactl using either positional or named parameters:

> sendReceiveJson('http://localhost:52178/wordCount', [text:'here are some words to be counted'])
[response:7, statusCode:200]
> sendReceiveJson(url:'http://localhost:52178/wordCount', request:[text:'here are some more words to be counted'])
[response:8, statusCode:200]

Jactl infers the type of the parameters from the actual Java parameter types of the implementation method so there is no need to specify a type for the parameters.

Note
The names that you register the parameters with using param("...") do not have to match the names in the Java static method but do have to be declared in the right order. The names will be used to validate any calls that are done using named arguments.

Default Values

When registering the parameter names you can also specify a default value for parameters that are optional. For example, the built-in String.asNum(base) method parses a String that could be in any base (up to 36). By default, it assumes base 10 but can be used to parse hex strings or binary strings etc.:

Jactl.method(STRING)
     .name("asNum")
     .param("base", 10)
     .impl(BuiltinFunctions.class, "stringAsNum")
     .register();

The param("base", 10) means that if no parameter is supplied it will be automatically filled in with 10.

Exception Handling

If you would like your method or function to indicate that a runtime error has occurred where it doesn’t make sense to continue execution of the script then you should throw a io.jactl.runtime.RuntimeError exception. There are two constructors of RuntimeError that can be used:

  public RuntimeError(String error, String source, int offset);
  public RuntimeError(String error, String source, int offset, Throwable cause);

The source and offset parameters refer to the source code and offset into the source code where the error occurs. In order to know where your method/function is being invoked in the source code, you just declare a String and int parameter in your static implementation method and Jactl will take care of populating these parameters with the values from where in the code the method/function was invoked. Don’t declare these using .param() since they are not explicit parameters that the script writer passes in.

Any actual parameters for the method/function should be declared in the implementation method after these String and int parameters. Note that for methods, the String and int parameters come after the object that the method is being invoked on.

Here is an example class showing a global function implementation and a method implementation that both need to throw RuntimeError exceptions:

public class ExampleFunctions {
  public static void registerFunctions(JactlEnv env) {
    Jactl.function()
         .name("exampleFunction")
         .param("param1", "default-value")
         .impl(ExampleFunctions.class, "exampleFunctionImpl")
         .register();
    
    Jactl.method(JactlType.ANY)
         .name("exampleMethod")
         .param("firstArg", -1)
         .impl(ExampleFunctions.class, "exampleMethodImpl")
         .register();
  }
  
  public static Object exampleFunctionImpl(String source, int offset, String param1) {
    try {
      ...
    }
    catch (Exception e) {
      throw new RuntimeError("Error invoking exampleFunction", source, offset, e);
    }
  }
  
  public static int exampleMethodImpl(Object obj, String source, int offset, int firstArg) {
    try {
      ...
    }
    catch (Exception e) {
      throw new RuntimeException("Error invoking exampleMethod", source, offset, e);
    }
  }
}

Suspending Execution

Some functions/methods need to perform long-running operations and wait for the operation to complete before returning a result. For example, the sendReceiveJson() function sends a request to a remote service and waits for the response before returning the response to the caller.

The problem is that we don’t want our sendReceiveJson() function to actually block and wait for the response. The response may take a long time, or we might time out waiting, and we don’t want to block the event-loop thread for that entire time. We want the function to suspend execution of the current script until the result is returned, freeing up the event-loop thread to process other events.

Any function that is going to suspend the current execution needs to accept a Continuation object as its first argument to allow the function to be able to be resumed/continued once the asynchronous operation has finished. The idea is that the function can check the Continuation object to see if it is being resumed (continued) after a suspension and can also obtain the result of the long-running asynchronous operation from the Continuation object if it is being resumed and then perform any further processing it needs to do after the asynchronous operation has returned a result. If the Continuation object is null it means that this is the first invocation and that it is not being resumed after a suspension.

There is no need to tell Jactl that the function/method is asynchronous as the presence of this Continuation parameter in the signature of the implementation method already indicates that the function/method can suspend execution.

For global functions that are asynchronous, the Continuation parameter of the function is always the first parameter. For methods that perform asynchronous operations, the Continuation parameters must be the second parameter after the parameter that represents the object on which the method is being invoked:

  public static Object someAsyncFunction(Continuation c, Object param1, String param2) { 
    ...
  }

  public static String someAsyncMethod(String obj, Continuation c, String param1, Object param2, int param3) {
    ...
  }

When performing asynchronous operations, there will invariably be errors that need to be thrown as exceptions, and so the usual signature for an asynchronous function will have the Continuation parameter followed by the String parameter for the source code, and the int parameter for the offset into the source where the invocation is taking place so the example above would more likely look like this:

  public static Object someAsyncFunction(Continuation c, String source, int offset, Object param1, String param2) { 
    ...
  }

  public static String someAsyncMethod(String obj, Continuation c, String source, int offset, String param1, Object param2, int param3) {
    ...
  }

In order to actually suspend the execution state, there are two static methods provided on the Continuation class:

1. Continuation.suspendBlocking(), and
2. Continuation.suspendNonBlocking().

The suspendBlocking() call is for use where the work to be done will be queued to occur on a blocking thread from the pool of blocking threads. Once a blocking thread is available it will run the work until completion and then schedule processing of the result back onto an event-loop thread (possibly the original event-loop thread if the execution environment provides that level of control).

The sendReceiveJson() function, for example, needs to send a request, wait for a response, and return the response to the caller. Since we can’t do this on an event-loop thread, we could schedule all of this to occur on a blocking thread using the Continuation.suspendBlocking() call like so:

  public static Map sendReceiveJson(Continuation c, String source, int offset, String url, Object request) {
    Continuation.suspendBlocking(() -> {
      try {
        // send request
        ...
        // wait for response
        ...
        return result;
      }
      catch (Exception e) {
        return new RuntimeError("Error invoking " + url, source, offset, e);   // Note: "return" not "throw"
      }
    });
  }

Note
Any errors that occur on the blocking thread should be returned (not thrown) as RuntimeError objects, as shown.

The use of Continuation.suspendBlocking() like this should be when the work being done requires using a blocking API and there is no away to avoid having thread blocked while the work is being performed. An interface to an SQL database might use this approach, for example, if the only API provided is a synchronous one.

In the case of the sendReceiveJson() function, however, since we are using Vert.x to do the send and receive, and Vert.x is by nature an asynchronous library, we don’t need to tie up an entire blocking thread while we wait for the response. Instead, we use the Continuation.suspendNonBlocking() and pass it a handler that will initiate the request and register its own handler with Vert.x to be invoked when the response is received.

Warning
It is important that the initiation of the async request be done inside the handler passed to Continuation.suspendNonBlocking() and not before, in order to guard against race conditions where the response might be received before we have finished suspending our execution.

The handler that we pass to Continuation.suspendNonBlocking() accepts two arguments:

1. a JactlContext in case we need it, and
2. a resume handler that will take care of resuming the execution and that must be passed the result once available.

In our example we don’t need the JactlContext so we just need to make sure that when the response handler we register with Vert.x is invoked we then invoke the resume handler we get passed in to us with the result. This will ensure that the script resumes on an event-loop thread (since Vert.x schedules response handlers on the same event-loop thread that the asynchronous operation was scheduled from) and that the result is handed back to the script.

A simple implementation of all this could look like this:

public class VertxFunctions {
  private static WebClient webClient;

  public static Map sendReceiveJson(Continuation c, String source, int offset, String url, Object request) {
    Continuation.suspendNonBlocking((context, resumer) -> {
      try {
        webClient.postAbs(url)
                 .sendJson(request)
                 .onSuccess(response -> {
                   // Our Vert.x response handler
                   resumer.accept(getResult(response));   // continue our script with result
                 });
      }
      catch (Exception e) {
        resumer.accept(new RuntimeError("Error invoking " + url, source, offset, e));   // Return error as result
      }
    });
    return null;       // never happens since function is async
  }
  
  ...
}

Note
Some details (such as better error handling) have been left out for brevity. To see the full implementation see the VertxFunctions example class.

Note
Any errors should be returned as RuntimeError objects to the resumer handler as a result as shown.

Both of the Continuation.suspendBlocking() and Coninuation.suspendNonBlocking() calls work by throwing a Continuation object. Each function in the call stack can then catch the Continuation and throw a new Continuation chained to the old one that captures any state they need to preserve. When continued after a resumption, the Continuation passed in will have this preserved state to allow the function to work out where it was up to and to continue with all the state it had before it was suspended.

Integration with REPL and CommandLine Scripts

For ease of integration with the Jactl REPL and Jactl commandline scripts, it is recommended that you have a public static function called registerFunctions(JactlEnv env) in one of your classes (generally the same one where the implementation is) that takes care of registering your functions. This allows you to configure the class name in your .jactlrc file and have the functions automatically available in the REPL and in commandline scripts.

For example:

class MyFunctions {
  public static registerFunctions(JactlEnv env) {
    Jactl.method(JactlType.ANY)
         .name("toJson")
         .impl(JsonFunctions.class, "toJson")
         .register();

   Jactl.method(JactlType.STRING)
        .name("fromJson")
        .impl(JsonFunctions.class, "fromJson")
        .register();
  }
  ...
}

See here for more details.

Async Parameters

Any function or method that suspends the current execution is known as asynchronous. Asynchronous functions and methods are flagged as being asynchronous by declaring a Continuation argument as discussed.

Sometimes a function or method is only asynchronous because it invokes something on one of its arguments that is asynchronous and could suspend the current execution. For example, a method that takes a closure/function as an argument and invokes it at some point has no way of knowing it the closure passed to it will suspend or not and so it has to assume that it can be suspended and declare a Continuation argument as discussed previously.

When the compiler detects that it is generating code to invoke a function that is potentially asynchronous it needs to generate some code to capture the current execution state. If the function or method is only asynchronous when one of its parameter values is an asynchronous function then sometimes it can be invoked with a value that is asynchronous, and sometimes it might be invoked with a value that isn’t. To prevent the compiler from having to generate unnecessary code for capturing execution state, the parameter can be flagged as being “async” and the compiler won’t generate this additional code if it knows that the argument being passed in cannot perform any asynchronous operations.

Parameters like these should be declared with asyncParam() instead of param() when registering the method or function. For example, if we had a function called measure() that invoked a passed in closure/function, and then measured how long it took to complete, it would be registered like this:

Jactl.function()
     .name("measure")
     .asyncParam("closure")     // only async when closure passed in is async
     .impl(MyFunctions.class, "measure")
     .register()

The implementation would look like this (closures/functions passed as arguments are always JactlMethodHandle objects):

class MyFunctions {
  public static long measure(Continuation c, String source, int offset, JactlMethodHandle closure) {
    
  }
  public static Object measureData;
}

Async Instance

For methods that act on JacsalType.ITERATOR objects, we allow the object to be one of the following types:

• List
• Map
• String — iterates of the characters of the string
• Number — iterates from 0 to n-1 (i.e. n times)
• Iterator — the result of methods such as map(), filter() etc

If the object is an actual Iterator then it could be in a chain of method calls where asynchronous operations are occurring. For example:

> [1,2,3].map{ sleep(1, it) + sleep(1, it) }.filter{ it != 2 }.each{ println it } 

In this example the object that filter() acts on is an Iterator and calling next() would result in the closure passed to map() being invoked which will suspend since it invokes sleep(). So even though filter() itself does not have any asynchronous arguments in this instance, it acts on an Iterator object that is asynchronous in nature. The asyncInstance(true) call when registering methods tells Jactl that a method is asynchronous when the object on which it acts is asynchronous.

So the registration of the filter() method looks like this:

Jactl.method(ITERATOR)
     .name("filter")
     .asyncInstance(true)
     .asyncParam("predicate", null)
     .impl(BuiltinFunctions.class, "iteratorFilter")
     .register();

This tells the compiler that the filter() call is only asynchronous if the object it acts on is asynchronous or the argument passed to it (the predicate closure) is asynchronous.

Note
As shown, asyncParam() calls are also allowed to specify a default value where appropriate.

Handling Resumption

Our sendReceiveJson() function from before did not have to consider how to resume itself after suspending because when the resumption occurred we already had the result so there was no more processing for the function to do.

We will now examine how our example measure() function might work to see how to handle functions that do need to do more processing when they are resumed.

A naive implementation of measure() might look like this:

class MyFunctions {
  public static void registerFunctions(JacsalEnv env) {
    Jactl.function()
         .name("measure")
         .param("closure")
         .impl(MyFunctions.class, "measure")
         .register();
  }
 
  public static long measure(String source, int offset, JactlMethodHandle closure) {
    long start = System.nanoTime();
    RuntimeUtils.invoke(closure, source, offset);
    return System.nanoTime() - start;
  }
  public static Object measureData;
}

We use the io.vertx.runtime.RuntimeUtils.invoke() helper method to invoke the closure. It gets passed the JactlMethodHandle, the source and offset, and then a varargs set of any arguments that the closure/function expects. In this case we are assuming a zero-arg closure/function was passed to us so there are no arguments that need to be passed in.

Once we have configured our .jactlrc file (see here for details) we can include our new function when running the Jactl REPL:

$ java -jar jactl-repl-2.0.0.jar
> long fib(long x) { x <= 2 ? 1 : fib(x-1) + fib(x-2) }
Function@1846982837
> measure{ fib(40) }
184077675

As you can see, it returns the number of nanoseconds it took to invoke fib(40) which equated to around 184ms.

Since the closure being invoked by measure() could suspend and wait for some asynchronous operation, measure() needs to gain control once it completes in order to work out how long it all took. In order to do that, we need to:

1. catch the Continuation thrown when the closure suspends itself,
2. create a new Continuation chained to the one it caught,
3. pass in a JactlMethodHandle to the new Continuation that points to a method that will be invoked when we are resumed,
4. capture any state we need in this new Continuation object (including the values of the parameters originally passed in if needed), and
5. throw the new Continuation.

The constructor of a Continuation looks like this:

public Continuation(Continuation      continuation,
                    JactlMethodHandle methodHandle,
                    int               codeLocation,
                    long[]            localPrimitives,
                    Object[]          localObjects)

The first parameter is the Continuation we just caught. This allows the Continuation objects to be chained together.

The second parameter is a JactlMethodHandle that will be invoked when we are resumed. It needs to point to a continuation method that takes a single Continuation argument. When resumed, it will be passed the Continuation we are constructing and throwing here.

The third parameter is an int called location which represents a logical location in our method where we were suspended. It allows us to record where in the method we were when we were suspended in case there are multiple locations where this can occur. We can pass in any value as long as we understand the relationship between the different values and the actual locations in the code.

The last two parameters are used to capture the current state. One is an array of long values, which we can use for storing any primitive values we need, and the other one is an array of Object values, where we store any non-primitive values we need.

In our case, the only state we need to capture is the start time which is a long so our code to catch a Continuation and then throw a new chained one would look like this:

  try {
    ...
  }
  catch(Continuation cont) {
    throw new Continuation(cont, measureResumeHandle, 0, new long[]{ start }, new Object[0]);
  }

We just pass location as a value of 0 for the moment since there is only one place where we can be suspended.

We need a continuation method that can be invoked when we are resumed so that we can create the JactlMethodHandle called measureResumeHandle that points to it and pass this handle to the Continuation on construction. The continuation method must take only a Continuation as an argument and must return Object.

In our case we want the resumption method to return the duration. In order to do that it needs to extract the value for the start time from our saved state in the Continuation. The Continuation has a field called localPrimitives for the long array and a field called localObject for the Object array that we passed in when we created it.

This means that our resumption method can look like this:

  public static Object measureResume(Continuation c) {
    long start = c.localPrimitives[0];
    return System.nanoTime() - start;
  }

To get a JactlMethodHandle we can use the utility method RuntimeUtils.lookupMethod(). This utility method takes a Class, the name of the static method, the return type, and then a varargs list of argument types.

Rather than invoke this every time it is better to do it once and store it in a static field for when we need it:

  private static JactlMethodHandle methodResumeHandle = RuntimeUtils.lookupMethod(MyFunctions.class, 
                                                                                  "measureResume",
                                                                                  Object.class,
                                                                                  Continuation.class);

We now need to make sure to tell Jactl that our function has an argument that makes us async if it is async by changing param() to asyncParam():

   Jactl.function()
        .name("measure")
        .asyncParam("closure")
        .impl(MyFunctions.class, "measure")
        .register();

We also need to add a Continuation parameter to our function since it is now potentially async:

  public static long measure(Continuation c, String source, int offset, JactlMethodHandle closure) {
    ...
  }

Putting this all together, our class now looks like this:

class MyFunctions {
  public static void registerFunctions(JacsalEnv env) {
    Jactl.function()
         .name("measure")
         .asyncParam("closure")
         .impl(MyFunctions.class, "measure")
         .register();
  }

  public static long measure(Continuation c, String source, int offset, JactlMethodHandle closure) {
    long start = System.nanoTime();
    try {
      RuntimeUtils.invoke(closure, source, offset);
      return System.nanoTime() - start;
    }
    catch(Continuation cont) {
     throw new Continuation(cont, measureResumeHandle, 0, new long[]{ start }, new Object[0]);
    }
  }
  public static Object measureData;

  public static Object measureResume(Continuation c) {
    long start = c.localPrimitives[0];
    return System.nanoTime() - start;
  }

  private static JactlMethodHandle measureResumeHandle = RuntimeUtils.lookupMethod(MyFunctions.class,
                                                                                   "measureResume",
                                                                                   Object.class,
                                                                                   Continuation.class);
}

Now we can measure how many nanoseconds it takes (wall clock time) for a closure to finish even if it does asynchronous operations:

> measure{ sleep(1000) }
1001947542

To illustrate a slightly more complicated scenario, imagine that we actually want to run the code we are measuring multiple times and return the average. To avoid having to duplicate code, our resume method should re-invoke our original method. The original method will then check if the Continuation argument is null or not to know whether it is the original call or a resumption of a previous call.

In order for the resume method to invoke the original method, it will need to be able to pass in values for source, offset, and closure, so we will need to store these as part of our state when throwing a Continuation, and we will use the location parameter to record which iteration we are up to.

Now our code looks like this:

class MyFunctions {
  public static void registerFunctions(JacsalEnv env) {
    Jactl.function()
         .name("measure")
         .param("count", 1)
         .asyncParam("closure")
         .impl(MyFunctions.class, "measure")
         .register();
  }

  public static long measure(Continuation c, String source, int offset, int count, JactlMethodHandle closure) {
    long start = c == null ? System.nanoTime() : c.localPrimitives[2];
    int  i     = c == null ? 0                 : c.methodLocation;
    try {
      for (; i < count; i++) {
        RuntimeUtils.invoke(closure, source, offset);
      }
      return (System.nanoTime() - start) / count;
    }
    catch(Continuation cont) {
      throw new Continuation(cont, measureResumeHandle,
                             i + 1,                      // location is next loop counter value
                             new long[]  { offset, count, start },
                             new Object[]{ source, closure });
    }
  }
  public static Object measureData;

  public static Object measureResume(Continuation c) {
    String source = (String)c.localObjects[0];
    int    offset = (int)c.localPrimitives[0];
    int count  = (int)c.localPrimitives[1];
    JactlMethodHandle closure = (JactlMethodHandle)c.localObjects[1];
    return measure(c, source, offset, count, closure);
  }

  private static JactlMethodHandle measureResumeHandle = RuntimeUtils.lookupMethod(MyFunctions.class,
                                                                                   "measureResume",
                                                                                   Object.class,
                                                                                   Continuation.class);
}

Now we invoke our measure() function with a count and a closure:

> long fib(long x) { x <= 2 ? 1 : fib(x-1) + fib(x-2) }
Function@1998137093
> measure(10){ fib(40) }
184077675

It will also do the right thing if we do some asynchronous operation (like a sleep()) inside the closure:

> measure(10){ sleep(100); fib(40) }
284705195

Of course, it is much easier to write this in Jactl itself since the language already takes care of suspending and resuming for you:

> def measure(n, closure) {
    def start = nanoTime()
    n.each{ closure() }
    (nanoTime() - start) / n
  }
Function@1230013344
> long fib(long x) { x <= 2 ? 1 : fib(x-1) + fib(x-2) }
Function@727860268
> measure(10){ sleep(100); fib(40) }
284375954

Example Application

In the jacsal-vertx project, an example application is provided that listens for JSON based web requests and runs a Jactl script based on the URI present in the request.

See Example Application for more details.