Note: information on this page refers to Ceylon 1.2, not to the current release.

Interoperation with Java

Ceylon is designed to execute in a virtual machine environment (loosely speaking, in an environment with built-in garbage collection), but it doesn't have its own native virtual machine. Instead, it "borrows" a virtual machine designed for some other language. For now the options are:

  • the Java Virtual Machine, or
  • a JavaScript virtual machine.

In this chapter, we're going to learn about how to interoperate with native code running on the Java Virtual Machine. In the next chapter we'll discuss JavaScript.

Depending on the Java SDK

A Ceylon program can only use native code belonging to a module. In particular, the Java SE SDK is available as a set of modules, according to the modularization proposed by the Jigsaw project. So to make use of the Java SE SDK we need to import one or more of these modules. For example:

  • the module java.base contains core packages including java.lang, java.util, and,
  • the module java.desktop contains the AWT and Swing desktop UI frameworks, and
  • java.jdbc contains the JDBC API.

So, if we need to use the Java collections framework in our Ceylon program, we need to create a Ceylon module that depends on java.base.

module org.jboss.example "1.0.0" {
    import java.base "7";

Now, we can simply import the Java class we're interested in and use it like any ordinary Ceylon class:

import java.util { HashMap }

void hashyFun() {
    value hashMap = HashMap<String,Object>();

TODO: instructions for using JavaFX here.

Depending on a Java archive

To make use of native code belonging to a packaged .jar archive, you have two options:

  • add the archive to a Ceylon module repository, along with JBoss Modules metadata defined in a module.xml or file, or
  • import the archive directly from a legacy Maven repository.

A module.xml or file specifies dependency information for a .jar.

  • The format of the file is documented here, and
  • the module.xml descriptor format is defined here.

If you're using Ceylon IDE, and you don't want to write the module.xml descriptor by hand, go to File > Export ... > Ceylon > Java Archive to Module Repository.

The Ceylon module architecture interoperates with Maven via Aether. You can find more information here.

Deploying Ceylon on OSGi

Ceylon is fully interoperable with OSGI, so that Ceylon modules:

  • can be deployed as pure OSGI bundles in an OSGI container out-of-the-box without any modification of the module archive file,
  • can embed additional OSGI metadata, to declare services for example,
  • can easily use OSGI standard services

This provides a great and straightforward opportunity to run Ceylon code inside a growing number of JEE application servers or enterprise containers that are based upon (or integrated with) OSGI.

Installing the Ceylon distribution / Ceylon SDK in an OSGI container

In order to be able to resolve and start Ceylon module archives (.car files) inside an OSGI container, you will first need to install, in the OSGI container, all the bundles of the Ceylon distribution and SDK.

These bundles are available in a dedicated location on the Ceylon language web site under various delivery forms:

  • OSGI bundle repositories (OBR and R5 XML), for Felix-based containers for example,
  • P2 repositories for Eclipse development or deployment to an Equinox container,
  • Zip archives for direct deployment inside containers,
  • Apache Karaf (aka JBoss Fuse) features

The OSGI interoperability reference gives more details about the URLs providing these packages, as well as how to install these bundles, for various containers

OSGI metadata management

Module archives generated for execution on the Java virtual machine already contain the OSGI metadata that can be deduced from the Ceylon module descriptor.

More precisely, for the following module descriptor:

native("jvm") module example.pureCeylon "3.1.0" {
    import ceylon.locale "1.2.2";
    import java.base "7";
    shared import "1.2.2";

the following OSGI metadata will be automatically generated in the module archive META-INF/MANIFEST.MF entry:

Bundle-SymbolicName: example.pureCeylon
Bundle-Version: 3.1.0.v20160311-1742
Export-Package: example.pureCeylon;version=3.1.0
Require-Bundle: com.redhat.ceylon.dist;bundle-version=1.2.2;visibility

The Ceylon module imports are translated into a Require-Bundle header, and shared imports are translated into a reexport directive.The java.base "7" import is translated into a Require-Capability header that requires the same Java version.

Note that the com.redhat.ceylon.dist bundle is systematically and implicitly added as a dependency to all generated Ceylon archives.

However it is also possible to customize the OSGI metadata of the generated archive in 2 ways:

  1. Additional manifest entries or resources (such as a desclarative service descriptor) can be added by simply adding resources as explained below.

  2. Module imports that correspond to dependencies provided by the OSGI container (such as the OSGI framework bundle or any bundle assumed to be available in the OSGI container) should be omitted from the generated Require-Bundle header. The --osgi-provided-bundles=<modules> option of the ceylon compile command allows specifying those modules. The same list can also be specified in the compiler section of the configuration file.

Generally, once omitted from the Require-Bundle header, the dependencies to provided bundles should be declared in an Import-Package header added in the MANIFEST.MF resource, as mentioned in point 1.

Ceylon metamodel registration

In order to be able to fully leverage the power of the Ceylon language, the metamodel should be initialized for each module used by a Ceylon application. It is automatically performed when running from the command line through the ceylon run command, but in an OSGi container it should be done when starting the Ceylon module. The Ceylon OSGI distribution provides an easy way to register the metamodel for a module, as well as for all the modules transitively imported.

This can be done by adding an OSGI activator class to the Ceylon module, that performs this metamodel registration. The com.redhat.ceylon.dist OSGI bundle, implicitly required by all Ceylon modules, already provides such an activator class: com.redhat.ceylon.dist.osgi.Activator

So if you don't need to do anything else in your OSGI Ceylon bundle at startup, you can simply add the following line to your MANIFEST.MF resource:

Bundle-Activator: com.redhat.ceylon.dist.osgi.Activator

Alternatively, if you need to perform some sort initialization in your Ceylon module bundle startup, you can also define an Activator class that will delegate to the default com.redhat.ceylon.dist.osgi.Activator. In this case, you'll need to follow these three steps.

First, explicitly import the com.redhat.ceylon.dist module in your module descriptor:

native("jvm") module example.withActivator "1.0.0" {
    shared import com.redhat.ceylon.dist "1.2.2";
    import java.base "7";

Next, add the following class to your module:

import com.redhat.ceylon.dist.osgi {
    DefaultActivator = Activator
import org.osgi.framework {

shared class Activator() extends DefaultActivator() {
    shared actual void start(BundleContext context) {
        // do module startup stuff
        super.start(context); // will perform the metamodel registering
    shared actual void stop(BundleContext context) {
        // do module shutdown stuff

Finally, Add the following line to your example/withActivator/ROOT/META-INF/MANIFEST.MF resource:

Bundle-Activator: example.withActivator.Activator

Interoperation with Java types

There's a handful of things to be aware of when writing Ceylon code that calls a Java class or interface.

Java primitive types are mapped to Ceylon types

You're never exposed to Java primitive types when calling a Java method or field from Ceylon. Instead:

  • boolean is represented by Ceylon's Boolean type,
  • char is represented by Ceylon's Character type,
  • long, int, and short are represented by Ceylon's Integer type,
  • byte is represented by Ceylon's Byte type,
  • double and float are represented by Ceylon's Float type, and
  • java.lang.String is represented by Ceylon's String type.


According to these rules, all conversions from a Java primitive to a Ceylon type are widening conversions, and are guaranteed to succeed at runtime. However, conversion from a Ceylon type to a Java primitive type might involve an implicit narrowing conversion. For example, if:

  • a Ceylon Integer is assigned to a Java int or short,
  • a Ceylon Float is assigned to a Java float, or if
  • a Ceylon UTF-32 Character is assigned to a Java 16-bit char

the assignment can fail at runtime, producing an exception.

Note: it is not a goal of Ceylon's type system to warn about operations which might result in numeric overflow. In general, almost any operation on a numeric type, including + or *, can result in numeric overflow.

Gotcha again!

There is no mapping between Java's wrapper classes like java.lang.Integer or java.lang.Boolean and Ceylon basic types, so these conversions must be performed explicitly by calling, for example, intValue() or booleanValue(), or by explicitly instantiating the wrapper class, just like you would do in Java when converting between a Java primitive type and its wrapper class.

Java array types are represented by special Ceylon classes

Since there are no primitively-defined array types in Ceylon, arrays are represented by special classes. These classes are considered to belong to the package java.lang. (Which belongs to the module java.base.)

  • boolean[] is represented by the class BooleanArray,
  • char[] is represented by the class CharArray,
  • long[] is represented by the class LongArray,
  • int[] is represented by the class IntArray,
  • short[] is represented by the class ShortArray,
  • byte[] is represented by the class ByteArray,
  • double[] is represented by the class DoubleArray,
  • float[] is represented by the class FloatArray, and, finally,
  • T[] for any object type T is represented by the class ObjectArray<T>.

We can obtain a Ceylon Array without losing the identity of the underlying Java array.

ByteArray javaByteArray = ByteArray(10);
Array<Byte> byteArray = javaByteArray.byteArray;

You can think of the ByteArray as the actual underlying byte[] instance, and the Array<Byte> as an instance of the Ceylon class Array that wraps the byte[] instance.

The module contains a raft of additional methods for working with these Java array types.

Null values are checked at runtime

Java types offer no information about whether a field or method can produce a null value, except in the very special case of a primitive type. Therefore, the compiler inserts runtime null value checks wherever Ceylon code calls a Java function that returns an object type, or evaluates a Java field of object type, and assigns the result to an non-optional Ceylon type.

In this example, no runtime null value check is performed, since the return value of System.getProperty() is assigned to an optional type:

import java.lang { System }

void printUserHome() {
    String? home = System.getProperty("user.home");

In this example, however, a runtime type check occurs when the return value of System.getProperty() is assigned to the non-optional type String:

import java.lang { System }

String getUserHome() {
    return System.getProperty("user.home");

The runtime check ensures that null can never unsoundly propagate from native Java code with unchecked null values into Ceylon code with checked null values, resulting in an eventual NullPointerException in Ceylon code far from the original call to Java.


The Ceylon compiler doesn't have any information that a Java method could return null, and so it won't warn you at compile time if you call a Java method that sometimes returns null.

Java properties are exposed as Ceylon attributes

A Java getter/setter pair will appear to a Ceylon program as a Ceylon attribute. For example:

import java.util { Calendar, TimeZone } 

void calendaryFun() {
    Calendar calendar = Calendar.instance;
    TimeZone timeZone = calendar.timeZone;
    Integer timeInMillis = calendar.timeInMillis;

If you want to call a Java setter method, assign a value to it using =:

calendar.timeInMillis = system.milliseconds;


Note that there are certain corner cases here which might be confusing. For example, consider this Java class:

public class Foo {
    public String getBar() { ... }
    public void setBar(String bar) { ... }
    public void setBar(String bar, String baz) { ... }

From Ceylon, this will appear as if it were defined like this:

shared class Foo {
    shared String bar { ... }
    assign bar { ... }
    shared void setBar(String bar, String baz) { ... }


  • to call the single-argument setter, use an assignment statement like = bar, but
  • to call the two-argument setter, use a method call like foo.setBar(bar,baz).


In pure Ceylon code, we almost always use declaration-site variance. However, this doesn't work when we interoperate with Java generic types with wildcards. Therefore, Ceylon supports use-site variance (wildcards).

  • List<out Object> has a covariant wildcard, and is equivalent to List<? extends Object> in Java, and
  • Topic<in Object> has a contravariant wildcard, and is equivalent to Topic<? super Object> in Java.

Wildcard types are unavoidable when interoperating with Java, and perhaps occasionally useful in pure Ceylon code. But we recommend avoiding them, except where there's a really good reason.


Since Ceylon 1.2.1 it is possible to use a java.lang.Iterable in a Ceylon for statement, like this:

import java.lang{JIterable=Iterable, JString=String}

JIterable<Object> objects = ...;
for (obj in objects) {
    // ...

Note that there is no special handling for type of the iterated elements: If the iterable contains Java Strings they're not magically transformed to Ceylon Strings:

JIterable<JString> strings = ...;
for (s in strings) {
    // s is a JString

Also note this is not supported when using an entry or tuple destructing iterator:

JIterable<String-String> stringPairs = ...;
for (key->item in stringPairs) {
    // not supported

In practice it is unusual to have a Java Iterable of Ceylon Entrys or Tuples.

Utility functions and classes

In the module you'll find a suite of useful utility functions and classes for Java interoperation. For example, there are classes that adapt between Ceylon collection types and Java collection types.

Converting between Iterables

An especially useful adaptor is CeylonIterable, which lets you iterate any Java Iterable from a for loop in Ceylon, or apply any of the usual operations of a Ceylon Iterable to it.

import java.util { JList=List, JArrayList=ArrayList }
import { CeylonIterable }


JList<String> strings = JArrayList<String>();

for (string in CeylonIterable(strings)) {

(Alternatively, we could have used CeylonList in this example.)

Similarly there are CeylonStringIterable, CeylonIntegerIterable, CeylonFloatIterable,CeylonByteIterable and CeylonBooleanIterable classes which as well as converting the iterable type also convert the elements from their Java types to the corresponding Ceylon type.

Getting a java.util.Class

Another especially useful function is javaClass, which obtains an instance of java.util.Class for a given type.

import { javaClass }
import java.lang { JClass=Class }

JClass<Integer> jc = javaClass<Integer>();

The functions javaClassFromInstance and javaClassFromDeclaration are also useful.

Java annotations

You can annotate a Ceylon class or interface with Java annotations. However, you must use an initial lowercase identifier to refer to the Java annotation.

For example:

import javax.persistence {

class Person(firstName, lastName, city) {

    id generatedValue
    column { name = "pid"; }
    value _id = 0;

    shared String firstName;
    shared String lastName;

    manyToOne { optional=false; }
    shared City city;

    string => "Person(``_id``, ``firstName`` ``lastName``, ````)";

Note that id here refers to the Java annotation javax.persistence.Id.


A Java annotation with @Target(ElementType.METHOD) may be applied to a getter or setter method on Java. Similarly, it may be applied to a Ceylon getter or setter. But it may not be applied it to a Ceylon reference declaration.

For example, given the following Java annotation:

public @interface Awesome {}

We may apply the annotation to a getter or setter:

awesome String name => _name;
awesome assign name => _name = name;

But the following is not legal:

awesome String name = "Trompon"; //compile error!

The reason for this restriction is that if either:

  • the Ceylon reference is variable, or
  • the Java annotation is also marked @Target(ElementType.FIELD),

then this code would be ambiguous.


Some Java frameworks and environments require metadata packaged in the META-INF or WEB-INF directory of the module archive, or sometimes even in the root directory of the module archive. We've already seen how to package resources in Ceylon module archives by placing the resource files in the modules' subdirectory of the resource directory, named resource by default.

Then, given a module named

  • resources to be packaged in the root directory of the module archive should be placed in resource/net/example/foo/ROOT/,
  • resources to be packaged in the META-INF directory of the module archive should be placed in resource/net/example/foo/ROOT/META-INF/, and
  • resources to be packaged in the WEB-INF directory of the module archive should be placed in resource/net/example/foo/ROOT/WEB-INF/.


Here's a couple of limitations to be aware of:

  • You can't call Java methods using the named argument syntax, since Java 7 doesn't expose the names of parameters at runtime (and Ceylon doesn't yet depend on features of Java 8).
  • You can't obtain a method reference, nor a static method reference, to an overloaded method.
  • Java generic types don't carry reified type arguments at runtime, so certain operations that depend upon reified generics (for example, some is tests) fail at runtime.

There's more ...

In a mixed Java/Ceylon project, you'll probably need to use a build system like Gradle, Maven, or Apache ant. Ceylon has plugins for each of these build systems.

Finally, we're going to learn about interoperation with languages like JavaScript with dynamic typing.