死鎖.md

死鎖

定義

• 常見情形之一，同步的嵌套。
• 開發中盡量不發生，但考試時可能遇到。

Example

public class DeadLockTest2 {

	public static final Object LOCK_A = new Object();//定義兩個鎖(對象)
	public static final Object LOCK_B = new Object();
}

public class DeadLockTest1 implements Runnable {
	
	private boolean flag;
	
	DeadLockTest1(boolean flag) {
		
		this.flag = flag;
	}
	
	public void run() {
		
		if(flag) {
			
			while(true) {//嘗試直到鎖上為止
				synchronized(DeadLockTest2.LOCK_A) {
					
					System.out.println(Thread.currentThread().getName() + " @ if @ LOCKA");
					
					synchronized(DeadLockTest2.LOCK_B) {
						
						System.out.println(Thread.currentThread().getName() + " @ if @ LOCK_B");
					}
				}
			}
			
		} else {
			
			while(true) {//嘗試直到鎖上為止
				synchronized(DeadLockTest2.LOCK_B) {
					
					System.out.println(Thread.currentThread().getName() + " @ else @ LOCK_B");
					
					synchronized(DeadLockTest2.LOCK_A) {
						
						System.out.println(Thread.currentThread().getName() + " @ else @ LOCK_A");
					}
				}
			}
		}
	}
}

public class DeadLockTest {

	public static void main(String[] args) {

		DeadLockTest1 a = new DeadLockTest1(true);
		DeadLockTest1 b = new DeadLockTest1(false);
		
		Thread t1 = new Thread(a);
		Thread t2 = new Thread(b);
		
		t1.start();
		t2.start();
	}
}

打印結果

Thread-1 @ else @ LOCK_B
Thread-0 @ if @ LOCKA
//鎖不上的狀態

---

public class DeadLock implements Runnable {
	
	private int num = 10;
	Object obj = new Object();//創建對象傳入同步代碼塊
	boolean flag = true;
	
	public void run() {
		
		if(flag) {
			while(true) {
				
				synchronized(obj) {//簡單來講，相當於一道鎖(同步鎖)，讓其他線程進不來
					
						show();
				}
			}
		} else {
			
			while(true) {
				
				this.show();//同步函數
			}
		}
	}
	
	public synchronized void show() {
		
		synchronized(obj) {
			
			if(num > 0) {
				
				try {
					
					Thread.sleep(10);//暫停十毫秒
				
				} catch (InterruptedException e) {
									
				}
				System.out.println(Thread.currentThread().getName() + " sale " + num--);
			}
		}
	}
}

public class DeadLockDemo {

	public static void main(String[] args) {
			
			DeadLock d = new DeadLock();//創建一個線程任務對象

			Thread d1 = new Thread(d);
			Thread d2 = new Thread(d);
			
			d1.start();
			
			try {
				
				Thread.sleep(10);//主函數暫停十毫秒
			
			} catch (InterruptedException e) {
								
			}
			
			d.flag = false;//讓 t2 讀 else 語句
			d2.start();
	}
}

打印結果

Thread-0 sale 10
//鎖不上的狀態