embed-languages.md

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Embedding Languages

• Compile and Run a Polyglot Application
• Define Guest Language Functions as Java Values
• Access Guest Languages Directly from Java
• Access Java from Guest Languages
• Lookup Java Types from Guest Languages
• Computed Arrays Using Polyglot Proxies
• Host Access
• Build Native Images from Polyglot Applications
• Code Caching Across Multiple Contexts
• Embed languages in Guest Languages
• Build a Shell for Many Languages
• Step Through with Execution Listeners
• Enterprise Sandbox Resource Limits

The GraalVM Polyglot API lets you embed and run code from guest languages in JVM-based host applications.

Throughout this section, you will learn how to create a host application in Java that runs on GraalVM and directly calls a guest language. You can use the tabs beneath each code example to choose between JavaScript, R, Ruby, and Python.

Ensure you set up GraalVM before you begin.

Compile and Run a Polyglot Application

GraalVM can run polyglot applications written in any language implemented with the Truffle language implementation framework. These languages are henceforth referenced as guest languages.

Complete the steps in this section to create a sample polyglot application that runs on GraalVM and demonstrates programming language interoperability.

1. Create a hello-polyglot project directory.

2. In your project directory, add a HelloPolyglot.java file that includes the following code: {% include snippet-tabs tab1type="java" tab1id="Hello_Polyglot_JS" tab1name="JavaScript" tab1path="embed/hello_polyglot_js.java" tab2type="java" tab2id="Hello_Polyglot_R" tab2name="R" tab2path="embed/hello_polyglot_R.java" tab3type="java" tab3id="Hello_Polyglot_Ruby" tab3name="Ruby" tab3path="embed/hello_polyglot_ruby.java" tab4type="java" tab4id="Hello_Polyglot_Python" tab4name="Python" tab4path="embed/hello_polyglot_python.java" %}

 In this code:

• import org.graalvm.polyglot.* imports the base API for the Polyglot API.
• import org.graalvm.polyglot.proxy.* imports the proxy classes of the Polyglot API, needed in later examples.
• Context provides an execution environment for guest languages. R currently requires the allowAllAccess flag to be set to true to run the example.
• eval evaluates the specified snippet of guest language code.
• The try with resource statement initializes the Context and ensures that it is closed after use. Closing the context ensures that all resources including potential native resources are freed eagerly. Closing a context is optional but recommended. Even if a context is not closed and no longer referenced it will be freed by the garbage collector automatically.

3. Run javac HelloPolyglot.java to compile HelloPolyglot.java with GraalVM.

4. Run java HelloPolyglot to run the application on GraalVM.

You now have a polyglot application that consists of a Java host application and guest language code that run on GraalVM. You can use this application with other code examples to demonstrate more advanced capabilities of the Polyglot API.

To use other code examples in this section, you simply need to do the following:

1. Add the code snippet to the main method of HelloPolyglot.java.

2. Compile and run your polyglot application.

Define Guest Language Functions as Java Values

Polyglot applications let you take values from one programming language and use them with other languages.

Use the code example in this section with your polyglot application to show how the Polyglot API can return JavaScript, R, Ruby, or Python functions as Java values.

{% include snippet-tabs tab1type="java" tab1id="Function_JS" tab1name="JavaScript" tab1path="embed/function_js.java" tab2type="java" tab2id="Function_R" tab2name="R" tab2path="embed/function_R.java" tab3type="java" tab3id="Function_Ruby" tab3name="Ruby" tab3path="embed/function_ruby.java" tab4type="java" tab4id="Function_Python" tab4name="Python" tab4path="embed/function_python.java" %}

 In this code:

• Value function is a Java value that refers to a function.
• The eval call parses the script and returns the guest language function.
• The first assertion checks that the value returned by the code snippet can be executed.
• The execute call executes the function with the argument 41.
• The asInt call converts the result to a Java int.
• The second assertion verifies that the result was incremented by one as expected.

Access Guest Languages Directly from Java

Polyglot applications can readily access most language types and are not limited to functions. Host languages, such as Java, can directly access guest language values embedded in the polyglot application.

Use the code example in this section with your polyglot application to show how the Polyglot API can access objects, numbers, strings, and arrays.

{% include snippet-tabs tab1type="java" tab1id="Access_JS" tab1name="JavaScript" tab1path="embed/access_js_from_java.java" tab2type="java" tab2id="Access_R" tab2name="R" tab2path="embed/access_R_from_java.java" tab3type="java" tab3id="Access_Ruby" tab3name="Ruby" tab3path="embed/access_ruby_from_java.java" tab4type="java" tab4id="Access_Python" tab4name="Python" tab4path="embed/access_python_from_java.java" %}

 In this code:

• Value result is an Object that contains three members: a number named id, a string named text, and an array named arr.
• The first assertion verifies that the return value can contain members, which indicates that the value is an object-like structure.
• The id variable is initialized by reading the member with the name id from the resulting object. The result is then converted to a Java int using asInt().
• The next assert verifies that result has a value of 42.
• The text variable is initialized using the value of the member text, which is also converted to a Java String using asString().
• The following assertion verifies the result value is equal to the Java String "42".
• Next the arr member that holds an array is read.
• Arrays return true for hasArrayElements. R array instances can have members and array elements at the same time.
• The next assertion verifies that the size of the array equals three. The Polyglot API supports big arrays, so the array length is of type long.
• Finally we verify that the array element at index 1 equals 42. Array indexing with polyglot values is always zero-based, even for languages such as R where indices start with one.

Access Java from Guest Languages

Polyglot applications offer bi-directional access between guest languages and host languages. As a result, you can pass Java objects to guest languages.

Use the code example in this section with your polyglot application to show how guest languages can access primitive Java values, objects, arrays, and functional interfaces.

To permit guest languages to access any public method or field of a Java object, set allowAllAccess(true) when the context is built. In this mode, the guest language code must be fully trusted, as it can access other not explicitly exported Java methods using reflection.

{% include snippet-tabs tab1type="java" tab1id="Access_Java_from_JS" tab1name="JavaScript" tab1path="embed/access_java_from_js.java" tab2type="java" tab2id="Access_Java_from_R" tab2name="R" tab2path="embed/access_java_from_R.java" tab3type="java" tab3id="Access_Java_from_Ruby" tab3name="Ruby" tab3path="embed/access_java_from_ruby.java" tab4type="java" tab4id="Access_Java_from_Python" tab4name="Python" tab4path="embed/access_java_from_python.java" %}

 In this code:

• The Java class MyClass has four public fields id, text, arr, and ret42. The fields are initialized with 42, "42", new int[]{1, 42, 3}, and lambda () -> 42 that always returns an int value of 42.
• The Java class MyClass is instantiated and exported with the name javaObj into the polyglot scope, which allows the host and guest languages to exchange symbols.
• A guest language script is evaluated that imports the javaObj symbol and assigns it to the local variable which is also named javaObj. To avoid conflicts with variables, every value in the polyglot scope must be explicitly imported and exported in the top-most scope of the language.
• The next two lines verify the contents of the Java object by comparing it to the number 42 and the string '42'.
• The third verification reads from the second array position and compares it to the number 42. Whether arrays are accessed using 0-based or 1-based indices depends on the guest language. Independently of the language, the Java array stored in the arr field is always accessed using translated 0-based indices. For example, in the R language, arrays are 1-based so the second array element is accessible using index 2. In the JavaScript and Ruby languages, the second array element is at index 1. In all language examples, the Java array is read from using the same index 1.
• The last line invokes the Java lambda that is contained in the field ret42 and compares the result to the number value 42.
• After the guest language script executes, validation takes place to ensure that the script returns a boolean value of true as a result.

Lookup Java Types from Guest Languages

In addition to passing Java objects to the guest language, it is possible to allow the lookup of Java types in the guest language.

Use the code example in this section with your polyglot application to show how guest languages lookup Java types and instantiate them.

{% include snippet-tabs tab1type="java" tab1id="Lookup_Java_from_JS" tab1name="JavaScript" tab1path="embed/lookup_java_from_js.java" tab2type="java" tab2id="Lookup_Java_from_R" tab2name="R" tab2path="embed/lookup_java_from_R.java" tab3type="java" tab3id="Lookup_Java_from_Ruby" tab3name="Ruby" tab3path="embed/lookup_java_from_ruby.java" tab4type="java" tab4id="Lookup_Java_from_Python" tab4name="Python" tab4path="embed/lookup_java_from_python.java" %}

 In this code:

• A new context is created with all access enabled (allowAllAccess(true)).
• A guest language script is evaluated.
• The script looks up the Java type java.math.BigDecimal and stores it in a variable named BigDecimal.
• The static method BigDecimal.valueOf(long) is invoked to create new BigDecimals with value 10. In addition to looking up static Java methods, it is also possible to directly instantiate the returned Java type., e.g., in JavaScript using the new keyword.
• The new decimal is used to invoke the pow instance method with 20 which calculates 10^20.
• The result of the script is converted to a host object by calling asHostObject(). The return value is automatically cast to the BigDecimal type.
• The result decimal string is asserted to equal to "100000000000000000000".

Computed Arrays Using Polyglot Proxies

The Polyglot API includes polyglot proxy interfaces that let you customize Java interoperability by mimicking guest language types, such as objects, arrays, native objects, or primitives.

Use the code example in this section with your polyglot application to see how you can implement arrays that compute their values lazily.

Note: The Polyglot API supports polyglot proxies either on the JVM or in Native Image.

{% include snippet-tabs tab1type="java" tab1id="Proxy_JS" tab1name="JavaScript" tab1path="embed/proxy_js.java" tab2type="java" tab2id="Proxy_R" tab2name="R" tab2path="embed/proxy_R.java" tab3type="java" tab3id="Proxy_Ruby" tab3name="Ruby" tab3path="embed/proxy_ruby.java" tab4type="java" tab4id="Proxy_Python" tab4name="Python" tab4path="embed/proxy_python.java" %}

 In this code:

• The Java class ComputedArray implements the proxy interface ProxyArray so that guest languages treat instances of the Java class like arrays.
• ComputedArray array overrides the method get and computes the value using an arithmetic expression.
• The array proxy does not support write access. For this reason, it throws an UnsupportedOperationException in the implementation of set.
• The implementation for getSize returns Long.MAX_VALUE for its length.
• The main method creates a new polyglot execution context.
• A new instance of the ComputedArray class is then exported using the name arr.
• The guest language script imports the arr symbol, which returns the exported proxy.
• The second element and the 1000000000th element is accessed, summed up, and then returned. Note that array indices from 1-based languages such as R are converted to 0-based indices for proxy arrays.
• The result of the language script is returned as a long value and verified.

For more information about the polyglot proxy interfaces, see the Polyglot API JavaDoc.

Host Access

The Polyglot API by default restricts access to certain critical functionality, such as file I/O. These restrictions can be lifted entirely by setting allowAllAccess to true.

Note: The access restrictions are currently only supported with JavaScript.

Controlling Access to Host Functions

It might be desireable to limit the access of guest applications to the host. For example, if a Java method is exposed that calls System.exit then the guest application will be able to exit the host process. In order to avoid accidentally exposed methods, no host access is allowed by default and every public method or field needs to be annotated with @HostAccess.Export explicitly.

{% include snippet-tabs tab1type="java" tab1id="ExplicitHostAccess_js" tab1name="JavaScript" tab1path="embed/explicit_access_java_from_js.java" %}

 In this code:

• The class Employee is declared with a field name of type String. Access to the getName method is explicitly allowed by annotating the method with @HostAccess.Export.
• The Services class exposes two methods, createEmployee and exitVM. The createEmployee method takes the name of the employee as an argument and creates a new Employee instance. The createEmployee method is annotated with @HostAccess.Export and therefore accessible to the guest application. The exitVM method is not explicitly exported and therefore not accessible.
• The main method first creates a new polyglot context in the default configuration, disallowing host access except for methods annotated with @HostAccess.Export.
• A new Services instance is created and put into the context as global variable services.
• The first evaluated script creates a new employee using the services object and returns its name.
• The returned name is asserted to equal the expected name John Doe.
• A second script is evaluated that calls the exitVM method on the services object. This fails with a PolyglotException as the exitVM method is not exposed to the guest application.

Host access is fully customizable by creating a custom HostAccess policy.

Controlling Host Callback Parameter Scoping

By default, a Value lives as long as the corresponding Context. However, it may be desireable to change this default behavior and bind a value to a scope, such that when execution leaves the scope, the value is invalidated. An example for such a scope are guest-to-host callbacks, where a Value may be passed as a callback parameter. We have already seen above how this works with the default HostAccess.EXPLICIT:

public class Services {
    Value lastResult;

    @HostAccess.Export
    public void callback(Value result) {
        this.lastResult = result;
    }

    String getResult() {
        return this.lastResult.asString();
    }
}

public static void main(String[] args) {
    Services s = new Services()
    try (Context context = Context.newBuilder().allowHostAccess(HostAccess.EXPLICIT).build()) {
        context.getBindings("js").putMember("services", s);
        context.eval("js", "services.callback('Hello from JS');");
        System.out.println(s.getResult());
    }
}

In this example, lastResult maintains a reference to the value from the guest is stored on the host and remains accessible until after the scope of callback() has ended.

However, this is not always desireable, as keeping the value alive may block resources unnecessarily or not reflect the behavior of ephemeral values correctly. For these cases, HostAccess.SCOPED can be used, which changes the default behavior for all callbacks, such that values that are passed as callback parameters are only valid for the duration of the callback.

To make the above code work with HostAccess.SCOPED, individual values passed as a callback parameters can be pinned to extend their validity until after the callback returns:

public class Services {
    Value lastResult;

    @HostAccess.Export
    void callback(Value result, Value notneeded) {
        this.lastResult = result;
        this.lastResult.pin();
    }

    String getResult() {
        return this.lastResult.asString();
    }
}

public static void main(String[] args) {
    Services s = new Services()
    try (Context context = Context.newBuilder().allowHostAccess(HostAccess.SCOPED).build()) {
        context.getBindings("js").putMember("services", s);
        context.eval("js", "services.callback('Hello from JS', 'foobar');");
        System.out.println(services.getResult());
    }
}

Alternatively, the entire callback method can opt out from scoping if annotated with @HostAccess.DisableMethodScope, maintaining regular semantics for all parameters of the callback:

public class Services {
    Value lastResult;
    Value metaInfo;

    @HostAccess.Export
    @HostAccess.DisableMethodScope
    void callback(Value result, Value metaInfo) {
        this.lastResult = result;
        this.metaInfo = metaInfo;
    }

    String getResult() {
        return this.lastResult.asString() + this.metaInfo.asString();
    }
}

public static void main(String[] args) {
    Services s = new Services()
    try (Context context = Context.newBuilder().allowHostAccess(HostAccess.SCOPED).build()) {
        context.getBindings("js").putMember("services", s);
        context.eval("js", "services.callback('Hello from JS', 'foobar');");
        System.out.println(services.getResult());
    }
}

Access Privilege Configuration

It is possible to configure fine-grained access privileges for guest applications. The configuration can be provided using the Context.Builder class when constructing a new context. The following access parameters may be configured:

• Allow access to other languages using allowPolyglotAccess.
• Allow and customize access to host objects using allowHostAccess.
• Allow and customize host lookup to host types using allowHostClassLookup. Allows the guest application to look up the host application classes permitted by the lookup predicate. For example, a Javascript context can create a Java ArrayList, provided that ArrayList is allowlisted by the classFilter and access is permitted by the host access policy: context.eval("js", "var array = Java.type('java.util.ArrayList')")
• Allow host class loading using allowHostClassLoading. Classes are only accessible if access to them is granted by the host access policy.
• Allow the creation of threads using allowCreateThread.
• Allow access to native APIs using allowNativeAccess.
• Allow access to IO using allowIO and proxy file accesses using fileSystem.

Note: Granting access to class loading, native APIs, or host I/O effectively grants all access, as these privileges can be used to bypass other access restrictions.

Build Native Images from Polyglot Applications

Polyglot embeddings can also be compiled ahead-of-time using Native Image. By default, no language is included if the Polyglot API is used. To enable guest languages, the --language:<languageId> (e.g., --language:js) option needs to be specified. Currently, it is required to set the --initialize-at-build-time option when building a polyglot native image. All examples on this page can be converted to native executables with the native-image builder.

The following example shows how a simple HelloWorld JavaScript application can be built using native-image:

javac HelloPolyglot.java
native-image --language:js --initialize-at-build-time -cp . HelloPolyglot
./HelloPolyglot

It should be mentioned that you can also include a guest language into the native image, but exclude the JIT compiler by passing the -Dtruffle.TruffleRuntime=com.oracle.truffle.api.impl.DefaultTruffleRuntime option to the builder. Be aware, the flag -Dtruffle.TruffleRuntime=com.oracle.truffle.api.impl.DefaultTruffleRuntime has to placed after all the Truffle language/tool options, so that it will override the default settings.

You can build the above example again but this time the created image will only contain the Truffle language interpreter (the GraalVM compiler will not be included in the image) by running:

native-image --language:js -Dtruffle.TruffleRuntime=com.oracle.truffle.api.impl.DefaultTruffleRuntime --initialize-at-build-time -cp . HelloPolyglotInterpreter

Configuring Native Host Reflection

Accessing host Java code from the guest application requires Java reflection in order to work. When reflection is used within a native image, the reflection configuration file is required.

For this example we use JavaScript to show host access with native executables. Copy the following code in a new file named AccessJavaFromJS.java.

import org.graalvm.polyglot.*;
import org.graalvm.polyglot.proxy.*;
import java.util.concurrent.*;

public class AccessJavaFromJS {

    public static class MyClass {
        public int               id    = 42;
        public String            text  = "42";
        public int[]             arr   = new int[]{1, 42, 3};
        public Callable<Integer> ret42 = () -> 42;
    }

    public static void main(String[] args) {
        try (Context context = Context.newBuilder()
                                   .allowAllAccess(true)
                               .build()) {
            context.getBindings("js").putMember("javaObj", new MyClass());
            boolean valid = context.eval("js",
                   "    javaObj.id         == 42"          +
                   " && javaObj.text       == '42'"        +
                   " && javaObj.arr[1]     == 42"          +
                   " && javaObj.ret42()    == 42")
               .asBoolean();
            System.out.println("Valid " + valid);
        }
    }
}

Copy the following code into reflect.json:

{% highlight java %} {% include embed/access_java_from_reflection_config.json %} {% endhighlight %}

Now you can create a native executable that supports host access:

javac AccessJavaFromJS.java
native-image --language:js --initialize-at-build-time -H:ReflectionConfigurationFiles=reflect.json -cp . AccessJavaFromJS
./accessjavafromjs

Note that in case assertions are needed in the image, the -H:+RuntimeAssertions option can be passed to native-image. For production deployments, this option should be omitted.

Code Caching Across Multiple Contexts

The GraalVM Polyglot API allows code caching across multiple contexts. Code caching allows compiled code to be reused and allows sources to be parsed only once. Code caching can often reduce memory consumption and warm-up time of the application.

By default, code is cached within a single context instance only. To enable code caching between multiple contexts, an explicit engine needs to be specified. The engine is specified when creating the context using the context builder. The scope of code sharing is determined by the engine instance. Code is only shared between contexts associated with one engine instance.

All sources are cached by default. Caching may be disabled explicitly by setting cached(boolean cached) to false. Disabling caching may be useful in case the source is known to only be evaluated once.

Consider the following code snippet as an example:

public class Main {
    public static void main(String[] args) {
        try (Engine engine = Engine.create()) {
            Source source = Source.create("js", "21 + 21");
            try (Context context = Context.newBuilder()
                .engine(engine)
                .build()) {
                    int v = context.eval(source).asInt();
                    assert v == 42;
            }
            try (Context context = Context.newBuilder()
                .engine(engine)
                .build()) {
                    int v = context.eval(source).asInt();
                    assert v == 42;
            }
        }
    }
}

In this code:


• import org.graalvm.polyglot.* imports the base API for the Polyglot API.
• Engine.create() creates a new engine instance with the default configuration.
• Source.create() creates a source object for the expression “21 + 21” with "js" language, which is the language identifier for JavaScript.
• Context.newBuilder().engine(engine).build() builds a new context with an explicit engine assigned to it. All contexts associated with an engine share the code.
• context.eval(source).asInt() evaluates the source and returns the result as Value instance.

Embed Guest languages in Guest Languages

The GraalVM Polyglot API can be used from within a guest language using Java interoperability. This can be useful if a script needs to run isolated from the parent context. In Java as a host language a call to Context.eval(Source) returns an instance of Value, but since we executing this code as part of a guest language we can use the language-specific interoperability API instead. It is therefore possible to use values returned by contexts created inside of a language, like regular values of the language. In the example below we can conveniently write value.data instead of value.getMember("data"). Please refer to the individual language documentation for details on how to interoperate with foreign values. More information on value sharing between multiple contexts can be found here.

Consider the following code snippet as an example:

import org.graalvm.polyglot.*;

public class Main {
    public static void main(String[] args) {
        try (Context outer = Context.newBuilder()
                                   .allowAllAccess(true)
                               .build()) {
            outer.eval("js", "inner = Java.type('org.graalvm.polyglot.Context').create()");
            outer.eval("js", "value = inner.eval('js', '({data:42})')");
            int result = outer.eval("js", "value.data").asInt();
            outer.eval("js", "inner.close()");

            System.out.println("Valid " + (result == 42));
        }
    }
}

In this code:


• Context.newBuilder().allowAllAccess(true).build() builds a new outer context with all privileges.
• outer.eval evaluates a JavaScript snippet in the outer context.
• inner = Java.type('org.graalvm.polyglot.Context').create() the first JS script line looks up the Java host type Context and creates a new inner context instance with no privileges (default).
• inner.eval('js', '({data:42})'); evaluates the JavaScript code ({data:42}) in the inner context and returns stores the result.
• "value.data" this line reads the member data from the result of the inner context. Note that this result can only be read as long as the inner context is not yet closed.
• context.eval("js", "c.close()") this snippet closes the inner context. Inner contexts need to be closed manually and are not automatically closed with the parent context.
• Finally the example is expected to print Valid true to the console.

Build a Shell for Many Languages

With just a few lines of code, the GraalVM Polyglot API lets you build applications that integrate with any guest language supported by GraalVM.

This shell implementation is agnostic to any particular guest language.

BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
PrintStream output = System.out;
Context context = Context.newBuilder().allowAllAccess(true).build();
Set<String> languages = context.getEngine().getLanguages().keySet();
output.println("Shell for " + languages + ":");
String language = languages.iterator().next();
for (;;) {
    try {
        output.print(language + "> ");
        String line = input.readLine();
        if (line == null) {
            break;
        } else if (languages.contains(line)) {
            language = line;
            continue;
        }
        Source source = Source.newBuilder(language, line, "<shell>")
                        .interactive(true).buildLiteral();
        context.eval(source);
    } catch (PolyglotException t) {
        if(t.isExit()) {
            break;
        }
        t.printStackTrace();
    }
}

Step Through with Execution Listeners

The GraalVM Polyglot API allows users to instrument the execution of guest languages through ExecutionListener class. For example, it lets you attach an execution listener that is invoked for every statement of the guest language program. Execution listeners are designed as simple API for polyglot embedders and may become handy in, e.g., single-stepping through the program.

import org.graalvm.polyglot.*;
import org.graalvm.polyglot.management.*;

public class ExecutionListenerTest {
    public static void main(String[] args) {
        try (Context context = Context.create("js")) {
            ExecutionListener listener = ExecutionListener.newBuilder()
                      .onEnter((e) -> System.out.println(
                              e.getLocation().getCharacters()))
                      .statements(true)
                      .attach(context.getEngine());
            context.eval("js", "for (var i = 0; i < 2; i++);");
            listener.close();
        }
    }
}

In this code:

• The Context.create() call creates a new context for the guest language.
• Create an execution listener builder by invoking ExecutionListeners.newBuilder().
• Set onEnter event to notify when element's execution is entered and consumed. At least one event consumer and one filtered source element needs to be enabled.
• To complete the listener attachment, attach() needs to be invoked.
• The statements(true) filters execution listeners to statements only.
• The context.eval() call evaluates a specified snippet of guest language code.
• The listener.close() closes a listener earlier, however execution listeners are automatically closed with the engine.

{% include_relative sandbox-options.md %}