How does the Handler mechanism work in Android

Handler is the core mechanism for inter-thread communication in Android, used to pass messages between worker threads and the main (UI) thread.

Core Components of Handler

1. Handler

• Purpose: Send and process messages

• Features:

  • Bound to the Looper of the thread that created it
  • Responsible for sending Messages to MessageQueue
  • Processes messages distributed by Looper

2. Looper

• Purpose: Message loop, continuously retrieves messages from MessageQueue

• Features:

  • Each thread can only have one Looper
  • Main thread has Looper created by default
  • Sub-threads need to call Looper.prepare() and Looper.loop() manually

3. MessageQueue

• Purpose: Queue for storing messages

• Features:

  • First In First Out (FIFO)
  • Sorted by time (when field)
  • Implemented using singly linked list

4. Message

• Purpose: Carrier for data and information

• Properties:

  • what: Message identifier
  • arg1/arg2: Integer parameters
  • obj: Object parameter
  • callback: Runnable callback

Handler Workflow

shell
Send message flow:
Handler.sendMessage() → MessageQueue.enqueueMessage() → Message enqueued

Process message flow:
Looper.loop() → MessageQueue.next() → Handler.dispatchMessage() → Handler.handleMessage()

Code Example

java
// Main thread Handler
Handler mainHandler = new Handler(Looper.getMainLooper()) {
    @Override
    public void handleMessage(Message msg) {
        // Process message, update UI
    }
};

// Sub-thread Handler
new Thread(() -> {
    Looper.prepare();
    Handler threadHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            // Process message
        }
    };
    Looper.loop();
}).start();

Handler Memory Leak

Cause:

• Non-static inner class holds reference to outer Activity
• Delayed messages prevent Activity from being recycled

Solutions:

1. Use static inner class + WeakReference
2. Remove all messages when Activity is destroyed
3. Use Handler's removeCallbacksAndMessages(null)

Key Points

• Main thread Looper is created in ActivityThread.main()
• Difference between Handler.post() and sendMessage()
• Message.obtain() reuses message objects to avoid memory allocation
• IdleHandler executes when message queue is idle