Dexmaker

A Java-language API for doing compile time or runtime code generation targeting the Dalvik VM. Unlike
cglib or ASM, this library creates Dalvik .dex
files instead of Java .class files.

It has a small, close-to-the-metal API. This API mirrors the
Dalvik bytecode specification giving you tight
control over the bytecode emitted. Code is generated instruction-by-instruction; you bring your own abstract
syntax tree if you need one. And since it uses Dalvik's dx tool as a backend, you get efficient register
allocation and regular/wide instruction selection for free.

What does it do?

Mockito Mocks

Dexmaker lets you use the Mockito mocking library in your
Android projects by generating Dalvik bytecode class proxies. Just add an
androidTestImplementation dependency on dexmaker-mockito and you can use Mockito in your Android Instrumentation tests.

The version of Mockito that Dexmaker targets can be found in dexmaker-mockito's build.gradle file. The general rule is that the major and minor version of Dexmaker will match the underlying major and minor version of Mockito.

Mocking Final Classes & Methods

Starting in Android "P", it is possible to mock final classes and methods using the dexmaker-mockito-inline library. If you execute your tests on a device or emulator running Android P or above, you can add an androidTestImplementation dependency on dexmaker-mockito-inline (instead of dexmaker-mockito; don't add both) and you can use the normal Mockito APIs to mock final classes and methods in your Android Instrumentation tests.

NOTE: This functionality requires OS APIs which were introduced in Android P and cannot work on older versions of Android.

Class Proxies

Dexmaker includes a stock code generator for class proxies.
If you just want to do AOP or class mocking, you don't need to mess around with bytecodes.

Runtime Code Generation

This example generates a class and a method. It then loads that class into the current process and invokes its method.

public final class HelloWorldMaker {
    public static void main(String[] args) throws Exception {
        DexMaker dexMaker = new DexMaker();

        // Generate a HelloWorld class.
        TypeId<?> helloWorld = TypeId.get("LHelloWorld;");
        dexMaker.declare(helloWorld, "HelloWorld.generated", Modifier.PUBLIC, TypeId.OBJECT);
        generateHelloMethod(dexMaker, helloWorld);

        // Create the dex file and load it.
        File outputDir = new File(".");
        ClassLoader loader = dexMaker.generateAndLoad(HelloWorldMaker.class.getClassLoader(),
                outputDir, outputDir);
        Class<?> helloWorldClass = loader.loadClass("HelloWorld");

        // Execute our newly-generated code in-process.
        helloWorldClass.getMethod("hello").invoke(null);
    }

    /**
     * Generates Dalvik bytecode equivalent to the following method.
     *    public static void hello() {
     *        int a = 0xabcd;
     *        int b = 0xaaaa;
     *        int c = a - b;
     *        String s = Integer.toHexString(c);
     *        System.out.println(s);
     *        return;
     *    }
     */
    private static void generateHelloMethod(DexMaker dexMaker, TypeId<?> declaringType) {
        // Lookup some types we'll need along the way.
        TypeId<System> systemType = TypeId.get(System.class);
        TypeId<PrintStream> printStreamType = TypeId.get(PrintStream.class);

        // Identify the 'hello()' method on declaringType.
        MethodId hello = declaringType.getMethod(TypeId.VOID, "hello");

        // Declare that method on the dexMaker. Use the returned Code instance
        // as a builder that we can append instructions to.
        Code code = dexMaker.declare(hello, Modifier.STATIC, Modifier.PUBLIC);

        // Declare all the locals we'll need up front. The API requires this.
        Local<Integer> a = code.newLocal(TypeId.INT);
        Local<Integer> b = code.newLocal(TypeId.INT);
        Local<Integer> c = code.newLocal(TypeId.INT);
        Local<String> s = code.newLocal(TypeId.STRING);
        Local<PrintStream> localSystemOut = code.newLocal(printStreamType);

        // int a = 0xabcd;
        code.loadConstant(a, 0xabcd);

        // int b = 0xaaaa;
        code.loadConstant(b, 0xaaaa);

        // int c = a - b;
        code.op(BinaryOp.SUBTRACT, c, a, b);

        // String s = Integer.toHexString(c);
        MethodId<Integer, String> toHexString
                = TypeId.get(Integer.class).getMethod(TypeId.STRING, "toHexString", TypeId.INT);
        code.invokeStatic(toHexString, s, c);

        // System.out.println(s);
        FieldId<System, PrintStream> systemOutField = systemType.getField(printStreamType, "out");
        code.sget(systemOutField, localSystemOut);
        MethodId<PrintStream, Void> printlnMethod = printStreamType.getMethod(
                TypeId.VOID, "println", TypeId.STRING);
        code.invokeVirtual(printlnMethod, null, localSystemOut, s);

        // return;
        code.returnVoid();
    }
}

Download

For Mockito support, download the latest .jar via Maven:

    <dependency>
      <groupId>com.linkedin.dexmaker</groupId>
      <artifactId>dexmaker-mockito</artifactId>
      <version>2.25.1</version>
      <type>pom</type>
    </dependency>

or Gradle:

    androidTestImplementation 'com.linkedin.dexmaker:dexmaker-mockito:2.25.1'

Note: The dependency on Mockito will be transitively included, so there's no need to specify both Mockito AND dexmaker-mockito

You can also download the files manually from Bintray if you prefer.