直接看源码吧:

TestInnerClass.java
package com.qihoo.browser.activity;

import android.os.Handler;
import android.os.Message;

public class TestInnerClass {
    final MyHandler mH = new MyHandler();
    private class MyHandler extends Handler {
        public void handleMessage(Message msg) {
            handleLaunchActivity();
        }
    }

    private void handleLaunchActivity() {
    Log.i("ahking","111");
    }

}

编译器对这个类编译后会产生三个.class文件, TestInnerClass.class, TestInnerClass$MyHandler.class 和TestInnerClass$1.class
可见， 编译器对内部类会生成一个独立的class文件.
把这三个class文件用 IntellijIdea 打开看看反编译的结果:

TestInnerClass$MyHandler.class 内部类对应的.class文件

package com.qihoo.browser.activity;

import android.os.Handler;
import android.os.Message;
import com.qihoo.browser.activity.TestInnerClass;

class TestInnerClass$MyHandler extends Handler {
    private TestInnerClass$MyHandler(TestInnerClass var1) {
        this.this$0 = var1;
    }

    public void handleMessage(Message msg) {
        TestInnerClass.access$100(this.this$0); //源代码: handleLaunchActivity();
    }
}

TestInnerClass.class 外部类对应的.class文件
package com.qihoo.browser.activity;

import android.os.Handler;
import android.os.Message;

public class TestInnerClass {
    final TestInnerClass.MyHandler mH = new TestInnerClass.MyHandler();

    public TestInnerClass() {
    }

    private void handleLaunchActivity() {
    }

    private class MyHandler extends Handler {
        private MyHandler() {
        }

        public void handleMessage(Message msg) {
            TestInnerClass.this.handleLaunchActivity(); //源代码: handleLaunchActivity();
        }
    }
}

TestInnerClass$1.class 一个中间的class文件， 对我们并没有实际意义.
package com.qihoo.browser.activity;

// $FF: synthetic class
class TestInnerClass$1 {
}

疑问出来了， 我们并没有找到 TestInnerClass.access$100(this.this$0);的定义， 这是因为IntellijIdea对class文件的反编译结果做了优化，所以没有了access$100的信息. 这个时候就需要借助javap这个原始反编译工具了.

wangxin@wangxin:~/src/browser_6.9.7_forcoopad$ javap -c ./out/production/browser/com/qihoo/browser/activity/TestInnerClass\$MyHandler.class
Compiled from "TestInnerClass.java"
class com.qihoo.browser.activity.TestInnerClass$MyHandler extends android.os.Handler {
  final com.qihoo.browser.activity.TestInnerClass this$0;

  public void handleMessage(android.os.Message);
    Code:
       0: aload_0       
       1: getfield      #2                  // Field this$0:Lcom/qihoo/browser/activity/TestInnerClass;
       4: invokestatic  #4                  // Method com/qihoo/browser/activity/TestInnerClass.access$100:(Lcom/qihoo/browser/activity/TestInnerClass;)V
       7: return        

  com.qihoo.browser.activity.TestInnerClass$MyHandler(com.qihoo.browser.activity.TestInnerClass, com.qihoo.browser.activity.TestInnerClass$1);
    Code:
       0: aload_0       
       1: aload_1       
       2: invokespecial #1                  // Method "<init>":(Lcom/qihoo/browser/activity/TestInnerClass;)V
       5: return        
}
//可见， 在内部类的handleMessage实现中，通过 invokestatic 调用了外部类的TestInnerClass.access$100方法, 和IntellijIdea给出的结论一致.

wangxin@wangxin:~/src/browser_6.9.7_forcoopad$ javap -c ./out/production/browser/com/qihoo/browser/activity/TestInnerClass.class
Compiled from "TestInnerClass.java"
public class com.qihoo.browser.activity.TestInnerClass {
  public com.qihoo.browser.activity.TestInnerClass$MyHandler mH;

  public com.qihoo.browser.activity.TestInnerClass();
    Code:
       0: aload_0       
       1: invokespecial #2                  // Method java/lang/Object."<init>":()V
       4: aload_0       
       5: new           #3                  // class com/qihoo/browser/activity/TestInnerClass$MyHandler
       8: dup           
       9: aload_0       
      10: aconst_null   
      11: invokespecial #4                  // Method com/qihoo/browser/activity/TestInnerClass$MyHandler."<init>":(Lcom/qihoo/browser/activity/TestInnerClass;Lcom/qihoo/browser/activity/TestInnerClass$1;)V
      14: putfield      #5                  // Field mH:Lcom/qihoo/browser/activity/TestInnerClass$MyHandler;
      17: return        

  static void access$100(com.qihoo.browser.activity.TestInnerClass);
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method handleLaunchActivity:()V
       4: return        
}
//编译器为外部类添加了一个static void access$100()方法， 方法内调用自己的handleLaunchActivity()方法.

另一个就是我们刚才提到的，没啥用的class文件.

wangxin@wangxin:~/src/browser_6.9.7_forcoopad$ javap -c ./out/production/browser/com/qihoo/browser/activity/TestInnerClass\$1.class
Compiled from "TestInnerClass.java"
class com.qihoo.browser.activity.TestInnerClass$1 {
}

打个断点看一下:

inner_class_code.png

inner_class_break.png

结论: 为了内部类能调用到外部类的某个方法， 会借助编译器生成的static void access$*()方法实现. 我们在平时开发过程中， 打断点在调用栈中发现"access$100()"这样的调用时也就不用迷惑这条调用的作用了.