The Dafny compilation process translates Dafny programs into target language source code, compiles those to Java .class files, and then potentially runs the result.

The Dafny-to-Java compiler writes out the translated files of a file A.dfy to a folder A-java. The -out option can be used to choose a different output folder. The file A.dfy is translated to, which is placed in the output folder along with helper files. If more than one .dfy file is listed on the command-line, then the output folder name is taken from the first file, and .java files are written for each of the .dfy files. A-java will also contain translations to java for any library modules that are used.

A multi-language program that combines Dafny and Java code “just” needs to be sure that the translated Dafny code fits in to the Java code. There are two aspects to this:

  • ensuring that the names of entities in the translated Dafny code are usable from Java
  • ensuring that the types are the same on both sides

The Dafny runtime library

The step of compiling Java files (using javac) requires the Dafny runtime library. That library is automatically included if dafny is doing the compilation, but not if dafny is only doing translation.

Manually executing Dafny-generated Java code

Suppose a Dafny program is contained in a .dfy files, A.dfy, including a Main method. One can build the corresponding Java program (without running it) using this command:

dafny build --target:java A.dfy

Alternatively, one can insert in A.dfy the directive include B.dfy and omit B.dfy from the command-line.

The compiled program is then executed using the command java -cp "A-java:A-java/DafnyRuntime.jar" A

Alternatively the build and run steps can be combined: `dafny run –target:java A.dfy

If there is more than one .dfy file, general practice is to use include directives to include dependencies.

Calling Java from Dafny

Calling a Java method from Dafny requires declaring a shim in Dafny that gives a name and types that can be referenced by Dafny code, while still being translated to the same name and types as in the Java code.

For example, suppose we want to call a Java method demo.Demo1.p(). In we have

package demo;
public class Demo1 {
  public static void p() { System.out.println("Hi!\n"); }

In Demo1.dfy we write,

module M {
  method {:extern "demo.Demo1", "p"} p() 
  method Main() {

Note that the declaration of p() has no body; it is just a declaration because the actual implementation is in Java. Its extern attribute has two arguments: the fully-qualified class name and the method name.

Then the above can be built with dafny build --target:java Demo1.dfy and then run as a Java program with java -cp Demo1-java:Demo1-java/DafnyRuntime.jar Demo1

Or, in one build-and-run step: dafny run --target:java Demo1.dfy --input

Calling non-static Java methods from Dafny

If the Java methods to be called are not static, then a receiver object must also be created and shared between Java and Dafny. The following example shows how to do this.

In a Demo1a.dfy file:

module {:extern "demo"} M {
  method {:extern "demo.Demo1a", "newDemo1a"} newDemo1a() returns (r: Demo1a )
  class {:extern "demo", "Demo1a" } Demo1a {
    method {:extern "demo.Demo1a", "p"} p()
module MM {
  import opened M
  method Main() {
    var d: Demo1a := newDemo1a();

In a file:

package demo;
public class Demo1a {
  public static Demo1a newDemo1a() { return new Demo1a(); }
  public void p() { System.out.println("Hi!\n"); }

In the Java file there are two methods: a static one that returns a new instance of the Demo1a class, and a non-static method that does something useful. In the .dfy file we have a module M that has an extern name corresponding to the package of the Java class, a class declaration that associates the Dafny class Demo1a with the Java class demo.Demo1a, an instance method declaration that connects the Dafny method M.Demo1a.p to the Java method demo.Demo1a.p, and a static Dafny method M.newDemo1a connected to the static Java method demo.Demo1a.newDemo1a. The extern declaration for class Demo1a is actually not necessary if the Dafny and Java classes have the same name and the Dafny class is contained in a module whose extern name is the same as the Java package.

Then the Main method in the Dafny file calls the two Dafny methods, which are translated to the two Java methods. The combination is built and run using dafny run --target:java Demo1a.dfy --input

Calling Dafny from Java

The simplest way to call a Dafny method from Java is to place the Dafny method in a class within a module. The module should be attributed as {:extern “M”} where M is the desired name for the Java package in which the class resides. There is no need to give an extern name to the class or the method. For example, given this Dafny code (in Demo2.dfy)

module {:extern "M"} M {
  class C {
    static method p() {
      print "Hello, World\n";

and this Java code (in

package demo;
public class Demo2 {
  public static void main(String... args) {

then the commands

  • dafny build --target:java Demo2.dfy
  • java -cp "Demo2-java:Demo2-java/DafnyRuntime.jar" demo.Demo2 compile and run the program. (Use ; instead of : on Windows systems.) Note that dafny run does not run the program as it does not know that there is a main method in the Java code.

If the Dafny method (say p()) is not a member of any class and is in the top-level (unnamed) module, then it is called as _System.__default.p()

Specifying class path

The dafny build and run steps invoke javac and java. Dafny will use the version of those tools that would be invoked at a terminal prompt (e.g., the ones found by a Linux $PATH). There is no mechanism to supply options to these internal uses of javac and java. However, dafny does use the value of the environment variable CLASSPATH. For example, if you are writing a Dafny program,, which calls methods in Demo.jar, you can build and run the program using CLASSPATH=`pwd`/Demo.jar dafny run --target:java Demo.dfy. The contents of the CLASSPATH must all be absolute paths.

There are two means to include other options in the build. First, one could create scripts named java and javac that invoke the actual java and javac with the desired options or environment settings, and then place theses scripts on the system $PATH.

Alternately, one can use dafny translate to create the .java translation of the Dafny program and then use regular Java build processes to combine the Dafny-generated Java code with other Java code.

Matching Dafny and Java types

If the Java method has input arguments or an output value, then the Dafny declaration must use corresponding types in the Dafny declaration, as shown in this table. Here, T' for a type parameter T indicates the Java type corresponding to a Dafny type T.

Dafny type Java type
bool boolean
int java.math.BigInteger
int64 long
int32 int
int16 short
int8 byte
char char
bitvector appropriate int-based type
ORDINAL java.math.BigInteger
real dafny.BigRational
string dafny.DafnySequence<? extends Character>
C, C? (for class, iterator C) (class) C
(trait) T (interface) T
datatype, codatatype (class) C
subset type same as base type
tuple dafny.Tuple_n_
array T’[]
seq dafny.DafnySequence<? extends T’>
set, iset dafny.DafnySet<? extends T’>
multiset dafny.DafnySet<? extends T’>
map<T,U>, imap<T,U> dafny.DafnyMap<? extends T’>
imap<T,U>, imap<T,U> dafny.DafnyMap<? extends T’>
function (arrow) types java.util.function.Function<T’,U’>

Either there is a direct match between the Dafny-generated Java types and the types used in the Java code (as listed in the table above), or some wrapper code is needed either on the Java side or on the Dafny side.

  • If Dafny is calling a Java method in some Java library and no alterations to the Java library are possible or desired, then the Dafny code must be written in such a way that the Dafny types will translate directly to the Java types. Often this requires defining a Dafny class that corresponds to the Java class.
  • Alternately, one can write a Java wrapper that will translate between the Dafny-generated (Java) types and the types used in the desired method. The wrapper will do appropriate conversions and call the desired method.