src/base/android/jni_generator/README.md

# 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

**Without Crazy Linker:**
 * Java->Native calls are exported from the shared library and lazily resolved
   by the runtime (via `dlsym()`).

**With Crazy Linker:**
 * Java->Native calls are explicitly registered with JNI on the native side.
   Explicit registration is necessary because crazy linker provides its own
   `dlsym()`, but JNI is hardcoded to use the system's `dlsym()`.
   * The logic to explicitly register stubs is generated by
     `jni_registration_generator.py`.
     * This script finds all native methods by scanning all source `.java` files
       of an APK. Inefficient, but very convenient.
   * Since `dlsym()` is not used in this case, we use a linker script to avoid
     the cost of exporting symbols from the shared library (refer to
     `//build/config/android:hide_all_but_jni_onload`).
 * `jni_registration_generator.py` exposes two registrations methods:
   * `RegisterNonMainDexNatives` - Registers native functions needed by multiple
     process types (e.g. Rendereres, GPU process).
   * `RegisterMainDexNatives` - Registers native functions needed only by the
     browser process.

### 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

 * Methods marked as `native` will have stubs generated for them that forward
   calls to C++ function (that you must write).
 * If the first parameter is a C++ object (e.g. `long mNativePointer`), then the
   bindings will automatically generate the appropriate cast and call into C++
   code (JNI itself is only C).

### Calling Native -> Java

 * Methods annotated with `@CalledByNative` will have stubs generated for them.
 * Just call the generated stubs defined in generated `.h` files.

### 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, make 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 (required only when using crazy linker).

Refer to [//build/config/android/rules.gni](https://cs.chromium.org/chromium/src/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`