Java中如何避免死锁呢？

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 Java中如何避免死锁呢？

下文笔者讲述java中避免死锁的方法分享,如下所示

避免死锁的方法:
    不使用synchronized这个显式的锁,而采用信号量控制资源,可被多少线程访问
    当设置资源只可被一个线程访问时,则此时为锁住状态，
	信号量可设置获取的超时时间，
	对无法成功获取,可进行重复尝试,或指定尝试次数后,也可立即退出

例:
使用信号量控制死锁

package com.java265.other;
import java.util.Date;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
public class Test16 {
	public static String obj1 = "java265.com-1";
	public static final Semaphore a1 = new Semaphore(1);
	public static String obj2 = "java265.com-2";
	public static final Semaphore a2 = new Semaphore(1);

	public static void main(String[] args) {
		LockAa a = new LockAa();
		new Thread(a).start();
		LockBb b = new LockBb();
		new Thread(b).start();
	}
}

class LockAa implements Runnable {
	public void run() {
	  try {
		 System.out.println(new Date().toString() + " LockA 开始执行");
		while (true) {
		 if (Test16.a1.tryAcquire(1, TimeUnit.SECONDS)) {
			System.out.println(new Date().toString() + " LockA 锁住 obj1");
			  if (Test16.a2.tryAcquire(1, TimeUnit.SECONDS)) {
				System.out.println(new Date().toString() + " LockA 锁住 obj2");
				Thread.sleep(60 * 1000); // do something
			  } else {
				System.out.println(new Date().toString() + "LockA 锁 obj2 失败");
			     }
		 } else {
			System.out.println(new Date().toString() + "LockA 锁 obj1 失败");
		}
			Test16.a1.release(); // 释放
			Test16.a2.release();
			Thread.sleep(1000); // 马上进行尝试，现实情况下do something是不确定的
		}
	 } catch (Exception e) {
			e.printStackTrace();
		}
	}
}

class LockBb implements Runnable {
	public void run() {
		try {
			System.out.println("java265.com " + new Date().toString() + " LockB 开始执行");
			while (true) {
				if (Test16.a2.tryAcquire(1, TimeUnit.SECONDS)) {
					System.out.println(new Date().toString() + " LockB 锁住 obj2");
					if (Test16.a1.tryAcquire(1, TimeUnit.SECONDS)) {
						System.out.println(new Date().toString() + " LockB 锁住 obj1");
						Thread.sleep(60 * 1000); // do something
					} else {
						System.out.println(new Date().toString() + "LockB 锁 obj1 失败");
					}
				} else {
					System.out.println(new Date().toString() + "LockB 锁 obj2 失败");
				}
				Test16.a1.release(); // 释放
				Test16.a2.release();
				Thread.sleep(10 * 1000); 
      // 这里只是为了演示，所以tryAcquire只用1秒，而且B要给A让出能执行的时间，否则两个永远是死锁
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

-----运行以上代码,将输出以下信息-----
Thu Apr 28 23:01:32 CST 2022 LockA 开始执行
java265.com Thu Apr 28 23:01:32 CST 2022 LockB 开始执行
Thu Apr 28 23:01:32 CST 2022 LockA 锁住 obj1
Thu Apr 28 23:01:32 CST 2022 LockB 锁住 obj2
Thu Apr 28 23:01:33 CST 2022LockB 锁 obj1 失败
Thu Apr 28 23:01:33 CST 2022LockA 锁 obj2 失败
Thu Apr 28 23:01:34 CST 2022 LockA 锁住 obj1
Thu Apr 28 23:01:34 CST 2022 LockA 锁住 obj2
Thu Apr 28 23:01:43 CST 2022 LockB 锁住 obj2
Thu Apr 28 23:01:43 CST 2022 LockB 锁住 obj1
Thu Apr 28 23:02:35 CST 2022 LockA 锁住 obj1
Thu Apr 28 23:02:35 CST 2022 LockA 锁住 obj2

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