LeakCanary原理

要观察一个对象是否泄漏，通过调用RefWatcher的watch方法，会创建一个KeyedWeakReference，并关联一个refenceQueue：

public void watch(Object watchedReference, String referenceName) {
    ......

    String key = UUID.randomUUID().toString();
    retainedKeys.add(key);
    final KeyedWeakReference reference =
        new KeyedWeakReference(watchedReference, key, referenceName, queue);

    watchExecutor.execute(new Runnable() {
      @Override public void run() {
        ensureGone(reference, watchStartNanoTime);
      }
    });
  }

在ensureGone里

void ensureGone(KeyedWeakReference reference, long watchStartNanoTime) {
    ....
    removeWeaklyReachableReferences();
    if (gone(reference) || debuggerControl.isDebuggerAttached()) {
      return;
    }
    gcTrigger.runGc();
    removeWeaklyReachableReferences();
    if (!gone(reference)) {
      long startDumpHeap = System.nanoTime();
      long gcDurationMs = NANOSECONDS.toMillis(startDumpHeap - gcStartNanoTime);

      File heapDumpFile = heapDumper.dumpHeap();

      if (heapDumpFile == HeapDumper.NO_DUMP) {
        // Could not dump the heap, abort.
        return;
      }
      long heapDumpDurationMs = NANOSECONDS.toMillis(System.nanoTime() - startDumpHeap);
      heapdumpListener.analyze(
          new HeapDump(heapDumpFile, reference.key, reference.name, excludedRefs, watchDurationMs,
              gcDurationMs, heapDumpDurationMs));
    }
  }

检查keyedweakreference是否被回收，如果被回收，返回，如果没被回收，触发一次gc，然后再次检查是否被回收，如果没被回收，说明是有泄漏了，做一次heapdump进行分析。

在heapdump中查找keyedweakreference，然后看keyedWeakreference的key是否与生成heapdump的key相同，然后分析leak 的最短路径。

从leakcanary的源代码中可以看出，gc root的类型

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