背景

GcTrigger DEFAULT = new GcTrigger() {
    @Override public void runGc() {
      // Code taken from AOSP FinalizationTest:
      // https://android.googlesource.com/platform/libcore/+/master/support/src/test/java/libcore/
      // java/lang/ref/FinalizationTester.java
      // System.gc() does not garbage collect every time. Runtime.gc() is
 

      // more likely to perfom a gc.
      Runtime.getRuntime().gc();
      enqueueReferences();
      System.runFinalization(); 
}

跟進

到底有什麼不一樣呢？
我看了手頭的4.2.2以及openjdk的原始碼：

public static void gc() {
        Runtime.getRuntime().gc();
}

System.gc()的實現就是呼叫Runtime.getRuntime().gc()，所以兩者是等價的。所以這裡是否是作者多慮了呢？我又看了一下5.0的原始碼，果然不一樣了：

/**
 * Whether or not we need to do a GC before running the finalizers.
 */
  private static boolean runGC;

  /**
   * If we just ran finalization, we might want to do a GC to free the finalized objects.
   * This lets us do gc/runFinlization/gc sequences but prevents back to back System.gc().
   */
 

  private static boolean justRanFinalization;


/**
    * Provides a hint to the VM that it would be useful to attempt
    * to perform any outstanding object finalization.
    */
    public static void runFinalization() {
        boolean shouldRunGC;
        synchronized(lock) {
            shouldRunGC = runGC;
            runGC = false;
        }
        if (shouldRunGC) {
            Runtime.getRuntime().gc();
        }
        Runtime.getRuntime().runFinalization();
        synchronized(lock) {
            justRanFinalization = true;
        }
    }


 /**
   * Indicates to the VM that it would be a good time to run the
   * garbage collector. Note that this is a hint only. There is no guarantee
   * that the garbage collector will actually be run.
   */
  public static void gc() {
      boolean shouldRunGC;
      synchronized(lock) {
          shouldRunGC = justRanFinalization;
          if (shouldRunGC) {
              justRanFinalization = false;
        } else {
            runGC = true;
          }
    }
   if (shouldRunGC) {
          Runtime.getRuntime().gc();
   } 
}

這樣改之後，單純呼叫System.gc()是不會觸發Runtime.getRuntime().gc()的。但是會把這次嘗試紀錄下來，等到下次呼叫System.runFinalization()時，會先執行這個Runtime.getRuntime().gc()。
這樣改後的影響至少有兩點：
1.單純呼叫System.gc()是不會觸發Runtime.getRuntime().gc()的，直到呼叫了System.runFinalization()
2.System.gc() -> System.gc() -> … -> System.gc() ->System.runFinalization()，最終只會呼叫一次Runtime.getRuntime().gc()

為什麼要這樣改呢？
找到了這個commit，是這樣描述的:

Avoid running Runtime.gc() until we need to run finalization.

This prevents excessive explicit GC which are called from apps to get
good GC behavior on Dalvik. Calling System.gc() does not help on ART
since GC for alloc is much rarer.

If running finalizers is requested following a System.gc we remember
that a GC was requested and perform it ahead of finalization.

Bug: 12004934

從這裡可以得到兩點資訊：
1.首先這是為了修復一個bug 12004934，具體什麼bug找不到了
2.其次在art模式下，直接呼叫gc的效果不大。至於為什麼，還沒有深入進去了解，這是ART相關的另外一個專題了，後面再詳細跟進。

回到開頭，leakcanary的作者在這裡直接用了Runtime.getRuntime().gc()的確是有理由的，但是這應該不是最好的方式，因為從這個提交的描述來看，連續呼叫Runtime.getRuntime().gc()可能存在bug。修改後的模式是gc / finalization / gc，雖然leakcanary這裡的使用不會有問題。但是我覺得我們自己使用的話，用System.gc() 配合 System.runFinalization()會比較好。