19. JavaFFI


The Java Foreign Function Interface (or Java FFI) is a feature which allows Fantom code to easily utilize normal Java libraries. The Java FFI is basically a mapping of the Java type system into the Fantom type system:

  • Java packages => Fantom pods
  • Java classes => Fantom classes
  • Java interfaces => Fantom mixins
  • Java fields => Fantom fields
  • Java methods => Fantom methods

Fantom was designed to run on the JVM, so mapping between the two worlds is fairly straightfoward with a high level of interoperability. However, there is a level of impedance mismatch between the two type systems. Features supported by the Java FFI:

  • Interop with any Java API except those which use multi-dimensional arrays
  • Static type checking
  • Call overloaded Java methods from Fantom
  • Construct Java classes using the Fantom constructor syntax
  • Extend Java classes (only one level of inheritance allowed right now)
  • Implement Java interfaces
  • Implicit coercion between Java primitives and sys::Int, sys::Float
  • Implicit coercion between one-dimensional Object arrays and sys::List
  • Direct mappings for Java one-dimensional primitive arrays
  • Fantom reflection support for Java members
  • Dynamic invoke support against Java classes

Features which are not yet available in Java FFI:

  • Multi-dimensional arrays
  • Subclassing a Java class more than one level deep
  • Attempting to override a Java overloaded method; this means you cannot subclass or extend from a type with abstract overloaded methods

Features which are not supported result in a compile time error.

Interop Summary

The following table summarizes the mapping of Java types to Fantom types:

Java Type            Fantom Type
-----------          -----------
foo.bar.Baz          [java]foo.bar::Baz
boolean              sys::Bool
byte                 sys::Int
short                sys::Int
char                 sys::Int
int                  sys::Int
long                 sys::Int
float                sys::Float
double               sys::Float
java.lang.Object     sys::Obj
java.lang.String     sys::Str
java.lang.Boolean    sys::Bool?
java.lang.Long       sys::Int?
java.lang.Double     sys::Float?
java.math.BigDecimal sys::Decimal
Foo[]                Foo[]  // sys::List parameterized with Foo
boolean[]            [java]fanx.interop::BooleanArray
byte[]               [java]fanx.interop::ByteArray
short[]              [java]fanx.interop::ShortArray
char[]               [java]fanx.interop::CharArray
int[]                [java]fanx.interop::IntArray
long[]               [java]fanx.interop::LongArray
float[]              [java]fanx.interop::FloatArray
double[]             [java]fanx.interop::DoubleArray
Foo[][]              // unsupported for both primitivies and Objects

Quick reference for mapping Java code to Fantom code:

Java                         Fantom
--------------------------   --------------------------
import javax.swing           using [java] javax.swing
import java.util.Map.Entry   using [java] java.util::Map$Entry as Entry
JFrame f = new JFrame(...)   f := JFrame(...)
array.length                 array.size
array[i]                     array[i]
array[i] = val               array[i] = val
int[] x = new int[5]         x := IntArray(5)

How it Works

The Java FFI does not use any special Fantom syntax. Java APIs are imported into the Fantom type system via the normal using statement with a special syntax for pod names. Java packages are mapped to Fantom pods by prefixing the string "[java]". For example the Java package javax.swing has the Fantom pod name of [java]javax.swing.

The Fantom compiler itself has no knowledge of Java libraries, rather it supports FFI plugins based on pods being prefixed with "[ffi]". In the case of the Java FFI, the compilerJava pod is the compiler plugin for importing the Java type system into Fantom.

Fantom code using the Java FFI results in a normal pod file compiled down into fcode. The only difference is that the fcode contains type and member references which are Java specific. This means that Fantom pods with Java FFI calls are not necessarily portable; attempting to use a Java FFI call in a JavaScript or .NET environment will fail.

Class Path - Compiler

At compile time, the Java FFI compiler attempts to resolve Java packages and classes using the following classpath:

  1. jars found in "sun.boot.class.path"
  2. {java}lib/rt.jar (only if step above fails to find anything)
  3. {java}lib/ext/*.jar
  4. {fan}lib/java/ext/*.jar
  5. {fan}lib/java/ext/{Env.platform}/*.jar
  6. jars found in "java.class.path" system property
  7. any pod in your dependency chain

You can test your system classpath to see what files and packages are discovered using this command:

fan compilerJava::ClassPath

Class Path - Runtime

The Fantom JVM runtime uses a classloader per pod. The same classpath used during compile time as described above is applied during runtime. The only difference is that the Fantom runtime delegates to the standard system classloader to resolve the classes from steps 1-3. Steps 4-7 are explicitly handled by FanClassLoader.

Detailed steps for Fantom classloading on JVM:

  1. Classfile is contained in the pod zip:
    1. Fantom type foo::Bar maps to precompiled classfile fan/foo/Bar.class"
    2. Java type acme.foo.Bar maps to "acme/foo/Bar.class"
  2. Fantom type foo::Bar is loaded as follows:
    1. first resolve pod file "foo.pod" via Env
    2. emit classfile from "fcode/Bar.fcode"
  3. Search for pre-compiled classfile in dependent pods
  4. Search extension jars in "{fan.home}/lib/java/ext"
  5. Search extension jars in "{fan.home}/lib/java/ext/{platform}" (see Env)
  6. System classloader which typically includes "{java.home}/lib/ext"

You can bundle Java code in a pod zip file and it will be visible to classes within your pod as well as to any pod that has a dependency on your pod. You can easily convert to a JAR file into a Fantom pod using a build script that includes the JAR as a resource file.


The special Java primitives boolean, long, and double are implicitly mapped to the Fantom types sys::Bool, sys::Int, and sys::Float respectively. The other Java primitives are mapped as follows:

byte    sys::Int
short   sys::Int
char    sys::Int
int     sys::Int
long    sys::Int
float   sys::Float

The special primitives above are not directly supported by the Fantom type system. Therefore you cannot use them as local variables or in field or method signatures. They are always coerced to/from their Fantom representation.


All Java objects maintain a Type represtation to provide Fantom style reflection. You can use a dynamic call to toClass to get the java.lang.Class of a sys::Type:

ArrayList#            // evaluates to sys::Type
ArrayList#->toClass   // evaluates to java.lang.Class

You can also use the fanx.interop.Interop class to convert between java.lang.Class of a sys::Type:

Interop.toFan(cls)     // evaluates to sys::Type
Interop.toJava(type)   // evaluates to java.lang.Class


Arrays of Objects are implicitly boxed/unboxed as Fantom sys::List. If you call a method which returns an array of Objects it is boxed into a Fantom list of the appropriate type. Likewise you pass a Fantom list of the appropriate type whenever a Java array is expected.

Primitive arrays are handled specially without any boxing/unboxing. They are represented in the Fantom type system via the special types:

boolean[]  [java]fanx.interop::BooleanArray
byte[]     [java]fanx.interop::ByteArray
short[]    [java]fanx.interop::ShortArray
char[]     [java]fanx.interop::CharArray
int[]      [java]fanx.interop::IntArray
long[]     [java]fanx.interop::LongArray
float[]    [java]fanx.interop::FloatArray
double[]   [java]fanx.interop::DoubleArray

The make, get, set, and size methods provide symbolic representations for working with primitive arrays. They are mapped directly to a single opcode in the Java bytecode:

int[] x = new int[4]  =>  x := IntArray(4)
x[2]                  =>  x[3]
x[3] = 5              =>  x[3] = 5
x.length              =>  x.size


Any Java API which uses reference types are mapped into the Fantom type system as nullable:

// Java methods
String foo(Object x, int y, Foo z)
void bar(String[] x) {}

// Fantom representation
Str? foo(Obj? x, Int y, Foo? z)
Void bar(Str?[]? x) {}

Note the case of String[] we assume that the entire array could be null or that any of the array items may be null, so the Fantom mapping is Str?[]?.

Overloaded Methods

One impedance mismatch between Java and Fantom is that Java permits a field and method to have the same name. Java also allows method overloading where multiple methods with the same name may be declared with different parameter types.

Fantom only allows a single definition of a slot for a given name. However when calling out to Java types the compiler will correctly resolve overloaded fields and methods. Let's consider this Java class:

class Foo
  String a;
  String a() { return a; }
  void a(String x) { a = x; }
  String b() { return a; }

In the class above a is overloaded by a field and two methods. Let's look at how Fantom code is resolved against the Java members:

foo.a        // lack of parens indicates field get
foo.a = "x"  // field set
foo.a()      // call Foo.a() method
foo.a("x")   // call Foo.a(String) method
foo.b()      // call Foo.b() method
foo.b        // call Foo.b() method - no ambiguity so we can omit parens

Resolving a call to an overloaded version of the method follows the same rules as the Java Language Specification. It is a compile time error if the arguments do not match any methods or if they match multiple methods ambiguously.


Under the covers Fantom treats Java constructors just like the Java VM treats them - as special methods with the name of <init>. The standard constructor syntax is used to invoke a Java constructor:

a := Date()
b := Date(millis)
c := Date(2008-1900, 11, 13) // crazy API to create 13-Dec-2008

The constructor call is resolved against the Java constructors using the same rules for resolving overloaded methods.


The Java FFI enables you to both extend from a Java class and implement Java interfaces. For example to create a subclass of java.util.Date:

using [java] java.util::Date as JDate
class FanDate : JDate
  new now() : super() {}
  new make(Int millis) : super(millis) {}

The standard inheritance syntax is used to extend and implement Java types.

The Fantom subclass must define how the Fantom constructors call the Java superclass constructors. This is done by calling super as an overloaded constructor (if the base class has multiple constructors). You may not use a this constructor chain.

Constructors for a Fantom class which subclasses from a Java class are emitted as true Java constructors. Therefore you may not declare two Fantom constructors with the same parameter types as it would result in duplicate Java constructors. For example the following code would result in a compile time error:

class FanDate : Date
  new now() : super() {}
  new epoch() : super(0) {}

Because of this difference between Fantom constructors versus Java constructors there is currently a restriction that you may only subclass from a Java class one level deep. You may not subclass from a Fantom class which itself subclasses from a Java class.

Another restriction: because a Fantom class may not override overloaded methods, you may not create a concrete Fantom class which inherits abstract overloaded methods.


When a Fantom type subclasses a Java type, you can override the Java methods using the normal Fantom syntax with the following restrictions:

  • cannot override static or final methods
  • cannot override a method overloaded by field of same name
  • cannot override a method overloaded by parameter types
  • cannot override a method which uses multi-dimensional arrays in its signature

Consider this example:

// Java class
class Java
  void alpha() {}
  void alpha(Java x) {}
  void beta(Java x) {}
  int gamma(String[] x) {}

In the example above, the method alpha is overloaded by parameter types, therefore it is not permissible for a Fantom class to override it. However we can override beta and gamma:

class Fantom : Java
  override Void beta(Java? x) {}
  override Int gamma(Str?[]? x) {}

When we override a Java method we use the Fantom type representation of the signature. In the gamma method we map the return type int to sys::Int and the argument type from String[] to the Fantom list Str[].

Inner Classes

Inner classes in Java source are formatted using a dot, but in the Java VM they are represented using the "$" dollar sign. We import inner classes into the Fantom type system using the Java VM name:

using [java] java.util::Map$Entry as Entry

Note that since the "$" is not a legal identifier char in Fantom, you must rename the Java inner class to a valid Fantom identifier with the as keyword.

Dynamic Calls

Normal Fantom reflection and dynamic invoke is available with Java classes. The Fantom runtime will correctly map reflective calls and dynamic invokes against overloaded methods. Because it is possible for a Java class to have both a field and method of the same name, Type.field and Type.method might return different results for the same name. If an attempt is made to use dynamic invoke on slot which has both a field and method of that name, then the method always hides the field.

Functions as Interfaces

Inner classes are often used in Java as a substitute for closures. The Java FFI allows you to use a function where an interface with one method is expected. For example the interface Runnable defines one method called run, so we can use a Fantom function whenever a Runnable is expected:

// Java API
void execute(Runnable r)

// Fantom code
execute |->| { echo("run!") }

The standard rules for coercion between Java and Fantom types apply for how the function implements the interface's method.


Java annotations are imported into the Fantom type system as a special case of facets. Fantom classes and slots may be annotated with a Java annotation using the standard facet syntax:

// Java annotations
public @interface AnnoA {}
public @interface AnnoB { String value(); }
public @interface AnnoC { int i();  String s(); }

// Fantom syntax
@AnnoB { value = "foo" }
@AnnoC { i = 34; s = "bar" }
class Fantom {}

Assuming the annotation has a runtime retention policy, annotations added to a Fantom type or slot are available for Java reflection. However, the annotation will not be reflected in the Fantom type system as a facet.

The following annotation element types are currently supported:

Java                     Fantom
----                     -------
boolean                  sys::Bool literal
byte, short, int, long   sys::Int literal
float, double            sys::Float literal
String                   sys::Str literal
Class                    sys::Type literal
enum                     Java FFI enum field access
arrays of above          sys::List literal of above