react-native/ReactAndroid/src/main/java/com/facebook/jni/HybridData.java

// Copyright 2004-present Facebook. All Rights Reserved.

package com.facebook.jni;

import android.util.Log;

import com.facebook.proguard.annotations.DoNotStrip;
import com.facebook.soloader.SoLoader;

/**
 * This object holds a native C++ member for hybrid Java/C++ objects.
 *
 * NB: THREAD SAFETY
 *
 * {@link #resetNative} deletes the corresponding native object synchronously on whatever thread
 * the method is called on. Otherwise, deletion will occur on the {@link DestructorThread}
 * thread.
 */
@DoNotStrip
public class HybridData {

  static {
    SoLoader.loadLibrary("fb");
  }

  @DoNotStrip
  private Destructor mDestructor = new Destructor(this);

  /**
   * To explicitly delete the instance, call resetNative().  If the C++
   * instance is referenced after this is called, a NullPointerException will
   * be thrown.  resetNative() may be called multiple times safely.  Because
   * the {@link DestructorThread} also calls resetNative, the instance will not leak if this is
   * not called, but timing of deletion and the thread the C++ dtor is called
   * on will be at the whim of the Java GC.  If you want to control the thread
   * and timing of the destructor, you should call resetNative() explicitly.
   */
  public synchronized void resetNative() {
    mDestructor.destruct();
  }

  /**
   * N.B. Thread safety.
   * If you call isValid from a different thread than {@link #resetNative()} then be sure to
   * do so while synchronizing on the hybrid. For example:
   * <pre><code>
   * synchronized(hybrid) {
   *   if (hybrid.isValid) {
   *     // Do stuff.
   *   }
   * }
   * </code></pre>
   */
  public boolean isValid() {
    return mDestructor.mNativePointer != 0;
  }

  public static class Destructor extends DestructorThread.Destructor {

    // Private C++ instance
    @DoNotStrip
    private long mNativePointer;

    Destructor(Object referent) {
      super(referent);
    }

    @Override
    void destruct() {
      // When invoked from the DestructorThread instead of resetNative,
      // the DestructorThread has exclusive ownership of the HybridData
      // so synchronization is not necessary.
      deleteNative(mNativePointer);
      mNativePointer = 0;
    }

    static native void deleteNative(long pointer);
  }
}