Overview

JNI (Java Native Interface) is the mechanism that enables Java code to call native functions, and native code to call Java functions.

• Native code calls into Java using apis from <jni.h>, which basically mirror Java's reflection APIs.
• Java code calls native functions by declaring body-less functions with the native keyword, and then calling them as normal Java functions.

jni_generator generates boiler-plate code with the goal of making our code:

1. easier to write, and
2. typesafe.

jni_generator uses regular expressions to parse .Java files, so don't do anything too fancy. E.g.:

• Classes must be either explicitly imported, or are assumed to be in the same package. To use java.lang classes, add an explicit import.
• Inner classes need to be referenced through the outer class. E.g.: void call(Outer.Inner inner)

The presense of any JNI within a class will result in ProGuard obfuscation for the class to be disabled.

Exposing Native Methods

Generally Java->Native calls are exported from the shared library and lazily resolved by the runtime (via dlsym()). There are a number of notable exceptions to this. See usage of jni_registration_generator.py in the codebase.

The jni_registration_generator.py exposes a registration function when using manual registation:

• RegisterNatives - Registers all native functions.

Exposing Java Methods

Java methods just need to be annotated with @CalledByNative. The generated functions can be put into a namespace using @JNINamespace("your_namespace").

Usage

Because the generator does not generate any source files, generated headers must not be #included by multiple sources. If there are Java functions that need to be called by multiple sources, one source should be chosen to expose the functions to the others via additional wrapper functions.

Calling Java -> Native

• Declare methods using a nested interface annotated with @NativeMethods.
• The JNI annotation processor generates a class named ${OriginalClassName}Jni with a get() method that returns an implementation of the annotated interface. The C++ function that it routes to is the same as if it would be in the legacy method.
• For each JNI method:
  • C++ stubs are generated that forward to C++ functions that you must write.
  • If the first parameter is a C++ object (e.g. long native${OriginalClassName}), then the bindings will generate the appropriate cast and call into C++ code.

To add JNI to a class:

1. Enable the JNI processor by adding to your android_library target:

   annotation_processor_deps = [ "//base/android/jni_generator:jni_processor" ]
   deps = [ "//base:jni_java" ]
   

2. Create a nested-interface annotated with @NativeMethods that contains the declaration of the corresponding static methods you wish to have implemented.
3. Call native functions using ${OriginalClassName}Jni.get().${method}
4. In C++ code, #include the header ${OriginalClassName}_jni.h. (The path will depend on the location of the generate_jni BUILD rule that lists your Java source code.) Only include this header from a single .cc file as the header defines functions. That .cc must implement your native code by defining non-member functions named JNI_${OriginalClassName}_${UpperCamelCaseMethod} for static methods and member functions named ${OriginalClassName}::${UpperCamelCaseMethod} for non-static methods. Member functions need be declared in the header file as well.

Example:

Java

class MyClass {
  // Cannot be private. Must be package or public.
  @NativeMethods
  /* package */ interface Natives {
    void foo();
    double bar(int a, int b);
    // Either the |MyClass| part of the |nativeMyClass| parameter name must
    // match the native class name exactly, or the method annotation
    // @NativeClassQualifiedName("MyClass") must be used.
    //
    // If the native class is nested, use
    // @NativeClassQualifiedName("FooClassName::BarClassName") and call the
    // parameter |nativePointer|.
    void nonStatic(long nativeMyClass);
  }

  void callNatives() {
    // MyClassJni is generated by the JNI annotation processor.
    // Storing MyClassJni.get() in a field defeats some of the desired R8
    // optimizations, but local variables are fine.
    Natives jni = MyClassJni.get();
    jni.foo();
    jni.bar(1,2);
    jni.nonStatic(mNativePointer);
  }
}

C++

#include "base/android/jni_android.h"
#include "<path to BUILD.gn>/<generate_jni target name>/MyClass_jni.h"

class MyClass {
public:
  void NonStatic(JNIEnv* env);
}

// Notice that unlike Java, function names are capitalized in C++.
// Static function names should follow this format and don't need to be declared.
void JNI_MyClass_Foo(JNIEnv* env) { ... }
void JNI_MyClass_Bar(JNIEnv* env, jint a, jint b) { ... }

// Member functions need to be declared.
void MyClass::NonStatic(JNIEnv* env) { ... }

Using the ‘native’ keyword

• The binding generator also looks for native JNI method declarations and generates stubs for them. This used to be the norm, but is now obsolete.
• If you have native methods that you don't want stubs generated for, you should add @JniIgnoreNatives to the class.

Testing Mockable Natives

1. Add the JniMocker rule to your test.
2. Call JniMocker#mock in a setUp() method for each interface you want to stub out.

JniMocker will reset the stubs during tearDown().

/**
 * Tests for {@link AnimationFrameTimeHistogram}
 */
@RunWith(BaseRobolectricTestRunner.class)
@Config(manifest = Config.NONE)
public class AnimationFrameTimeHistogramTest {
    @Rule
    public JniMocker mocker = new JniMocker();

    @Mock
    AnimationFrameTimeHistogram.Natives mNativeMock;

    @Before
    public void setUp() {
        MockitoAnnotations.initMocks(this);
        mocker.mock(AnimationFrameTimeHistogramJni.TEST_HOOKS, mNativeMock);
    }

    @Test
    public void testNatives() {
        AnimationFrameTimeHistogram hist = new AnimationFrameTimeHistogram("histName");
        hist.startRecording();
        hist.endRecording();
        verify(mNativeMock).saveHistogram(eq("histName"), any(long[].class), anyInt());
    }
}

If a native method is called without setting a mock in a unit test, an UnsupportedOperationException will be thrown.

Special case: DFMs

DFMs have their own generated GEN_JNIs, which are <module_name>_GEN_JNI. In order to get your DFM's JNI to use the <module_name> prefix, you must add your module name into the argument of the @NativeMethods annotation.

So, for example, say your module was named test_module. You would annotate your Natives interface with @NativeMethods("test_module"), and this would result in test_module_GEN_JNI.

Testing for readiness: use get()

JNI Generator automatically produces checks that verify that the Natives interface can be safely called. These checks are compiled out of Release builds, making these an excellent way to determine whether your code is called safely.

Most of the time you would write your code so that you only use JNI once the native libraries are loaded. There's nothing extra you need to do here.

If you expect your code to be called by an external caller, it‘s often helpful to know ahead of time that the context is valid (ie. either native libraries are loaded or mocks are installed). In this case it is helpful to call get() method, that performs all the Debug checks listed above, but does not instantiate a new object for interfacing Native libraries. Note that the unused value returned by the get() method will be optimized away in release builds so there’s no harm in ignoring it.

Addressing Jni.get() exceptions.

When you identify a scenario leading to an exception, relocate (or defer) the appropriate call to be made to a place where (or time when) you know the native libraries have been initialized (eg. onStartWithNative, onNativeInitialized etc).

Please avoid calling LibraryLoader.isInitialized() / LibraryLoader.isLoaded() in new code. Using LibraryLoader calls makes unit-testing more difficult:

• this call can not verify whether Mock object is used, making the use of mocks more complicated,
• using LibraryLoader.setLibrariesLoadedForNativeTests() alters the state for subsequently executed tests, inaccurately reporting flakiness and failures of these victim tests.
• Introducing LibraryLoader.is*() calls in your code immediately affects all callers, forcing the authors of the code up the call stack to override LibraryLoader internal state in order to be able to unit-test their code.

Calling Native -> Java

• Methods annotated with @CalledByNative will have stubs generated for them.
  • Inner class methods must provide the inner class name explicitly (ex. @CalledByNative("InnerClassName"))
• Just call the generated stubs defined in generated .h files.
• For test-only methods you want to call from native, use @CalledByNativeForTesting which will ensure that it is stripped in our release binaries.

Java Objects and Garbage Collection

All pointers to Java objects must be registered with JNI in order to prevent garbage collection from invalidating them.

For Strings & Arrays - it's common practice to use the //base/android/jni_* helpers to convert them to std::vectors and std::strings as soon as possible.

For other objects - use smart pointers to store them:

• ScopedJavaLocalRef<> - When lifetime is the current function's scope.
• ScopedJavaGlobalRef<> - When lifetime is longer than the current function's scope.
• JavaObjectWeakGlobalRef<> - Weak reference (do not prevent garbage collection).
• JavaParamRef<> - Use to accept any of the above as a parameter to a function without creating a redundant registration.

Additional Guidelines / Advice

Minimize the surface API between the two sides. Rather than calling multiple functions across boundaries, call only one (and then on the other side, call as many little functions as required).

If a Java object “owns” a native one, store the pointer via "long mNativeClassName". Ensure to eventually call a native method to delete the object. For example, have a close() that deletes the native object.

The best way to pass “compound” types across in either direction is to create an inner class with PODs and a factory function. If possible, mark all the fields as “final”.

Build Rules

• generate_jni - Generates a header file with stubs for given .java files
• generate_jar_jni - Generates a header file with stubs for a given .jar file
• generate_jni_registration - Generates a header file with functions to register native-side JNI methods.

Refer to //build/config/android/rules.gni for more about the GN templates.

Changing jni_generator

• Python unit tests live in jni_generator_tests.py
• A working demo app exists as //base/android/jni_generator:sample_jni_apk