Introduction

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

Throughout this section, you 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. See Get Started.

Alternatively, you can have a look at the reference documentation in Javadoc:

  • The Polyglot Package allows you to configure and run polyglot applications.
  • The Proxy Package allows you to mimic guest language objects using proxies.
  • The IO Package allows you to customize the file system access of languages.

Compile and Run a Polyglot Application

Polyglot applications run code written in any language implemented with the Truffle Language Implementation Framework. These languages are henceforth referenced as guests 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:

  // COMPILE-CMD: javac {file}
// RUN-CMD: java {file}
// BEGIN-SNIPPET
import org.graalvm.polyglot.*;
import org.graalvm.polyglot.proxy.*;
// END-SNIPPET

public class hello_polyglot_js {

static
// BEGIN-SNIPPET
public class HelloPolyglot {
    public static void main(String[] args) {
        System.out.println("Hello Java!");
        try (Context context = Context.create()) {
            context.eval("js", "print('Hello JavaScript!');");
        }
    }
}
// END-SNIPPET

    public static void main(String[] args) {
        HelloPolyglot.main(null);
    }
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java {file}
// BEGIN-SNIPPET
import org.graalvm.polyglot.*;
import org.graalvm.polyglot.proxy.*;
// END-SNIPPET

public class hello_polyglot_R {

static
// BEGIN-SNIPPET
public class HelloPolyglot {
    public static void main(String[] args) {
        System.out.println("Hello Java!");
        try (Context context = Context.newBuilder()
                                   .allowAllAccess(true)
                               .build()) {
            context.eval("R", "print('Hello R!');");
        }
    }
}
// END-SNIPPET

    public static void main(String[] args) {
        HelloPolyglot.main(null);
    }
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java {file}
// BEGIN-SNIPPET
import org.graalvm.polyglot.*;
import org.graalvm.polyglot.proxy.*;
// END-SNIPPET

public class hello_polyglot_ruby {

static
// BEGIN-SNIPPET
public class HelloPolyglot {
    public static void main(String[] args) {
        System.out.println("Hello Java!");
        try (Context context = Context.create()) {
            context.eval("ruby", "puts 'Hello Ruby!'");
        }
    }
}
// END-SNIPPET

    public static void main(String[] args) {
        HelloPolyglot.main(null);
    }
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java {file}
// BEGIN-SNIPPET
import org.graalvm.polyglot.*;
import org.graalvm.polyglot.proxy.*;
// END-SNIPPET

public class hello_polyglot_python {

static
// BEGIN-SNIPPET
public class HelloPolyglot {
    public static void main(String[] args) {
        System.out.println("Hello Java!");
        try (Context context = Context.create()) {
            context.eval("python", "print('Hello Python!')");
        }
    }
}
// END-SNIPPET

    public static void main(String[] args) {
        HelloPolyglot.main(null);
    }
}

  

 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.

  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.*;

public class function_js {
    public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.create()) {
    Value function = context.eval("js", "x => x+1");
    assert function.canExecute();
    int x = function.execute(41).asInt();
    assert x == 42;
}
// END-SNIPPET
    }
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.*;

public class function_R {
    public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.newBuilder()
                           .allowAllAccess(true)
                       .build()) {
    Value function = context.eval("R", "function(x) x + 1");
    assert function.canExecute();
    int x = function.execute(41).asInt();
    assert x == 42;
}
// END-SNIPPET
    }
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.*;

public class function_ruby {
    public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.create()) {
    Value function = context.eval("ruby", "proc { |x| x + 1 }");
    assert function.canExecute();
    int x = function.execute(41).asInt();
    assert x == 42;
}
 // END-SNIPPET
    }
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.*;

public class function_python {
    public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.create()) {
    Value function = context.eval("python", "lambda x: x + 1");
    assert function.canExecute();
    int x = function.execute(41).asInt();
    assert x == 42;
}
 // END-SNIPPET
    }
}

  

 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.

  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.*;

public class access_js_from_java {
    public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.create()) {
    Value result = context.eval("js", 
                    "({ "                   +
                        "id   : 42, "       +
                        "text : '42', "     +
                        "arr  : [1,42,3] "  +
                    "})");
    assert result.hasMembers();

    int id = result.getMember("id").asInt();
    assert id == 42;

    String text = result.getMember("text").asString();
    assert text.equals("42");

    Value array = result.getMember("arr");
    assert array.hasArrayElements();
    assert array.getArraySize() == 3;
    assert array.getArrayElement(1).asInt() == 42;
}
// END-SNIPPET
    }
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.*;

public class access_R_from_java {
    public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.newBuilder()
                           .allowAllAccess(true)
                       .build()) {
    Value result = context.eval("R", 
                    "list("                +
                        "id   = 42, "      +
                        "text = '42', "    +
                        "arr  = c(1,42,3)" +
                    ")");
    assert result.hasMembers();
    
    int id = result.getMember("id").asInt();
    assert id == 42;
    
    String text = result.getMember("text").asString();
    assert text.equals("42");
    
    Value array = result.getMember("arr");
    assert array.hasArrayElements();
    assert array.getArraySize() == 3;
    assert array.getArrayElement(1).asInt() == 42;
}
// END-SNIPPET
    }
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.*;

public class access_ruby_from_java {
    public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.create()) {
    Value result = context.eval("ruby", 
                    "o = Struct.new(:id, :text, :arr).new(" +
                        "42, "       +
                        "'42', "     +
                        "[1,42,3] "  +
                    ")");
    assert result.hasMembers();
    
    int id = result.getMember("id").asInt();
    assert id == 42;
    
    String text = result.getMember("text").asString();
    assert text.equals("42");
    
    Value array = result.getMember("arr");
    assert array.hasArrayElements();
    assert array.getArraySize() == 3;
    assert array.getArrayElement(1).asInt() == 42;
}
// END-SNIPPET
    }
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.*;

public class access_python_from_java {
    public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.create()) {
    Value result = context.eval("python", 
                    "type('obj', (object,), {" +
                        "'id'  : 42, "         +
                        "'text': '42', "       +
                        "'arr' : [1,42,3]"     +
                    "})()");
    assert result.hasMembers();
    
    int id = result.getMember("id").asInt();
    assert id == 42;
    
    String text = result.getMember("text").asString();
    assert text.equals("42");
    
    Value array = result.getMember("arr");
    assert array.hasArrayElements();
    assert array.getArraySize() == 3;
    assert array.getArrayElement(1).asInt() == 42;
}
// END-SNIPPET
    }
}

  

 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. A later section describes how to run less trusted code with the Polyglot API.

  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import java.util.concurrent.Callable;
import org.graalvm.polyglot.*;

public class access_java_from_js {

// BEGIN-SNIPPET
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();
        assert valid == true;
    }
}
// END-SNIPPET
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import java.util.concurrent.Callable;
import org.graalvm.polyglot.*;

public class access_java_from_R {

// BEGIN-SNIPPET
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("R").putMember("javaObj", new MyClass());
        boolean valid = context.eval("R",
               "    javaObj$id         == 42"   +
               " && javaObj$text       == '42'" +
               " && javaObj$arr[[2]]   == 42"   +
               " && javaObj$ret42()    == 42")
           .asBoolean();
        assert valid == true;
    }
}
// END-SNIPPET
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import java.util.concurrent.Callable;
import org.graalvm.polyglot.*;

public class access_java_from_ruby {

// BEGIN-SNIPPET
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.getPolyglotBindings().putMember("javaObj", new MyClass());
        boolean valid = context.eval("ruby",
               "javaObj = Polyglot.import('javaObj')\n" +
               "    javaObj[:id]         == 42"         +
               " && javaObj[:text]       == '42'"       +
               " && javaObj[:arr][1]     == 42"         +
               " && javaObj[:ret42].call == 42")
           .asBoolean();
        assert valid == true;
    }
}
// END-SNIPPET
}
 
  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import java.util.concurrent.Callable;
import org.graalvm.polyglot.*;

public class access_java_from_python {

// BEGIN-SNIPPET
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.getPolyglotBindings().putMember("javaObj", new MyClass());
        boolean valid = context.eval("python",
               "import polyglot \n"                            +
               "javaObj =  polyglot.import_value('javaObj')\n" +
               "javaObj['id']                == 42"            +
               " and javaObj['text']         == '42'"          +
               " and javaObj['arr'][1]       == 42"            +
               " and javaObj['ret42'].call() == 42")
           .asBoolean();
        assert valid == true;
    }
}
// END-SNIPPET
}

  

 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 field arr 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.

  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.Context;

public class lookup_java_from_js {


public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.newBuilder()
                           .allowAllAccess(true)
                       .build()) {
    java.math.BigDecimal v = context.eval("js",
            "var BigDecimal = Java.type('java.math.BigDecimal');" +
            "BigDecimal.valueOf(10).pow(20)")
        .asHostObject();
    assert v.toString().equals("100000000000000000000");
}
// END-SNIPPET
}
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.Context;

public class lookup_java_from_R {


public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.newBuilder()
                           .allowAllAccess(true)
                       .build()) {
    java.math.BigDecimal v = context.eval("R",
            "BigDecimal = java.type('java.math.BigDecimal');\n" + 
            "BigDecimal$valueOf(10)$pow(20)")
        .asHostObject();
    assert v.toString().equals("100000000000000000000");
}
// END-SNIPPET
}
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.Context;

public class lookup_java_from_ruby {

public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.newBuilder()
                           .allowAllAccess(true)
                       .build()) {
    java.math.BigDecimal v = context.eval("ruby",
            "BigDecimal = Java.type('java.math.BigDecimal')\n" + 
            "BigDecimal.valueOf(10).pow(20)")
        .asHostObject();
    assert v.toString().equals("100000000000000000000");
}
// END-SNIPPET
}
}
 
  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.Context;

public class lookup_java_from_python {


public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.newBuilder()
                           .allowAllAccess(true)
                       .build()) {
    java.math.BigDecimal v = context.eval("python",
            "import java\n" +
            "BigDecimal = java.type('java.math.BigDecimal')\n" + 
            "BigDecimal.valueOf(10).pow(20)")
        .asHostObject();
    assert v.toString().equals("100000000000000000000");
}
// END-SNIPPET
}
}

  

 In this code:

  • A new context is created with all access enabled (allowAllAccess(true)).
  • A new context is created with all access enabled (allowAllAccess(true)).
  • A guest language script is evaluated.
  • The script looks the Java type java.math.BigDecimal up 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 a native image executable.

  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.*;
import org.graalvm.polyglot.proxy.*;

public class proxy_js {

// BEGIN-SNIPPET
static class ComputedArray implements ProxyArray {
    public Object get(long index) {
        return index * 2;
    }
    public void set(long index, Value value) {
        throw new UnsupportedOperationException();
    }
    public long getSize() {
        return Long.MAX_VALUE;
    }
}

public static void main(String[] args) {
    try (Context context = Context.create()) {
        ComputedArray arr = new ComputedArray();
        context.getBindings("js").putMember("arr", arr);
        long result = context.eval("js",
                    "arr[1] + arr[1000000000]")
                .asLong();
        assert result == 2000000002L;
    }
}
// END-SNIPPET
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.Context;
import org.graalvm.polyglot.Value;
import org.graalvm.polyglot.proxy.ProxyArray;

public class proxy_R {

// BEGIN-SNIPPET
static class ComputedArray implements ProxyArray {
    public Object get(long index) {
        return index * 2;
    }
    public void set(long index, Value value) {
        throw new UnsupportedOperationException();
    }
    public long getSize() {
        return Long.MAX_VALUE;
    }
}

public static void main(String[] args) {
    try (Context context = Context.newBuilder()
                               .allowAllAccess(true)
                           .build()) {
        ComputedArray arr = new ComputedArray();
        context.getPolyglotBindings().putMember("arr", arr);
        long result = context.eval("R",
               "arr <- import('arr');" +
               "arr[2] + arr[1000000001]")
           .asLong();
        assert result == 2000000002L;
    }
}
// END-SNIPPET
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.Context;
import org.graalvm.polyglot.Value;
import org.graalvm.polyglot.proxy.ProxyArray;

public class proxy_ruby {

// BEGIN-SNIPPET
static class ComputedArray implements ProxyArray {
    public Object get(long index) {
        return index * 2;
    }
    public void set(long index, Value value) {
        throw new UnsupportedOperationException();
    }
    public long getSize() {
        return Long.MAX_VALUE;
    }
}

public static void main(String[] args) {
    try (Context context = Context.newBuilder()
                               .allowAllAccess(true)
                           .build()) {
        ComputedArray arr = new ComputedArray();
        context.getPolyglotBindings().putMember("arr", arr);
        long result = context.eval("ruby",
               "arr = Polyglot.import('arr') \n" +
               "arr[1] + arr[1000000000]")
           .asLong();
        assert result == 2000000002L;
    }
}
// END-SNIPPET
}

  
  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.Context;
import org.graalvm.polyglot.Value;
import org.graalvm.polyglot.proxy.ProxyArray;

public class proxy_python {

// BEGIN-SNIPPET
static class ComputedArray implements ProxyArray {
    public Object get(long index) {
        return index * 2;
    }
    public void set(long index, Value value) {
        throw new UnsupportedOperationException();
    }
    public long getSize() {
        return Long.MAX_VALUE;
    }
}

public static void main(String[] args) {
    try (Context context = Context.newBuilder()
                               .allowAllAccess(true)
                           .build()) {
        ComputedArray arr = new ComputedArray();
        context.getPolyglotBindings().putMember("arr", arr);
        long result = context.eval("python",
               "import polyglot\n" +
               "arr = polyglot.import_value('arr') \n" +
               "arr[1] + arr[1000000000]")
           .asLong();
        assert result == 2000000002L;
    }
}
// END-SNIPPET
}

  

 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.

Run Less Trusted Code

The Polyglot API supports running less trusted code from a third-party source. By default a polyglot context evaluates code in a less trusted mode. If the code is fully trusted, then allowAllAccess can be set to true in order to avoid any further configuration.

Important: Running less trusted code is currently only supported with JavaScript.

Configuring Host Access

Running less trusted applications requires special attention to which host methods are exposed to the guest application. 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.

  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import org.graalvm.polyglot.Context;
import org.graalvm.polyglot.HostAccess;
import org.graalvm.polyglot.PolyglotException;

public class explicit_access_java_from_js {

static
// BEGIN-SNIPPET
public class Employee {
    private final String name;
    Employee(String name) {this.name = name;}

    @HostAccess.Export
    public String getName() {
        return name;
    }
}//END-SNIPPET
static//BEGIN-SNIPPET
public class Services {
    @HostAccess.Export
    public Employee createEmployee(String name) {
        return new Employee(name);
    }
    
    public void exitVM() {
        System.exit(1);
    }
}

public static void main(String[] args) {
    try (Context context = Context.create()) {
        Services services = new Services();
        context.getBindings("js").putMember("services", services);
        String name = context.eval("js",
                "let emp = services.createEmployee('John Doe');" + 
                "emp.getName()").asString();
        assert name.equals("John Doe");
        
        try {
            context.eval("js", "services.exitVM()");
            assert false;
        } catch (PolyglotException e) {
            assert e.getMessage().endsWith(
                    "Unknown identifier: exitVM");
        }
    }
}
// END-SNIPPET
}

  

 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 new polyglot context in default configuration. It therefore evaluates code in less trusted mode.
  • 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.

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:

Important: Allowing access to class loading, native APIs or host IO must not be enabled for less trusted code as these privileges effectively allow all access.

Reliable Timeouts

The Polyglot API lets you reliably cancel guest language execution after a timeout interval. Use the code example in this section with your polyglot application to see how less trusted code, e.g. an infinite while loop, can be reliably canceled.

  // COMPILE-CMD: javac {file}
// RUN-CMD: java -ea {file}
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.Callable;
import org.graalvm.polyglot.*;

public class timeout_js {

public static void main(String[] args) {
// BEGIN-SNIPPET
try (Context context = Context.create()) {
    context.initialize("js");

    Timer timer = new Timer(true);
    timer.schedule(new TimerTask() {
        @Override
        public void run() {
            context.close(true);
        }
    }, 1000);

    try {
        String lessTrustedCode = "while(true);";
        context.eval("js", lessTrustedCode);
        assert false;
    } catch (PolyglotException e) {
        assert e.isCancelled();
    }
}
// END-SNIPPET
}

}

  

 In this code:

  • The first line of code creates a new context for any language.
  • The second line of code ensures the language is initialized. This prevents the execution from being canceled during context initialization.
  • A new java.util.Timer instance is created.
  • To close the context, a new TimerTask is scheduled to run after one second. The true parameter to close indicates that the execution should be canceled if it is currently running.
  • An infinite while loop is evaluated. This code will never complete. For this reason, the timer task closes and cancels the execution. This scenario causes a PolyglotException to be thrown.
  • Finally, in the exception catch block, the exception is verified as originating from a cancel event.

Build Native Images from Polyglot Applications

Polyglot embeddings can also be compiled using GraalVM 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) native image option needs to be specified. All examples on this page are also supported when compiled using Native Image Generator – the native-image utility. Currently it is required to set the --initialize-at-build-time option when building with a polyglot language. We plan to lift this restriction in future versions.

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

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 images. 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);
        }
    }
}

Next, copy the following code into reflect.json:

[
  { "name": "AccessJavaFromJS$MyClass", "allPublicFields": true },
  { "name": "java.util.concurrent.Callable", "allPublicMethods": true }
]

Now the a native image can be created 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 enabling code caching across multiple contexts. Code caching allows compiled code to be reused and allows sources to be parsed only once. Often, code caching can reduce memory consumption and warmup time of the application.

By default, code is cached within a single context instance only. In order 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.

import org.graalvm.polyglot.*;

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.

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 that is available by default with 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

GraalVM Polyglot API allows 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 to detect less trusted code by counting the number of statements executed.

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.