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synchronized与Lock的区别

程序员文章站 2024-01-08 08:26:34
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两者区别:

1.首先synchronized是java内置关键字,在jvm层面,Lock是个java类;
2.synchronized无法判断是否获取锁的状态,Lock可以判断是否获取到锁;
3.synchronized会自动释放锁(a 线程执行完同步代码会释放锁 ;b 线程执行过程中发生异常   会释放锁),Lock需在finally中手工释放锁(unlock()方法释放锁),否则容易造成线程死锁;
4.用synchronized关键字的两个线程1和线程2,如果当前线程1获得锁,线程2线程等待。如果线程1阻塞,线程2则会一直等待下去,而Lock锁就不一定会等待下去,如果尝试获取不到锁,线程可以不用一直等待就结束了;
5.synchronized的锁可重入、不可中断、非公平,而Lock锁可重入、可判断、可公平(两者皆可)
6.Lock锁适合大量同步的代码的同步问题,synchronized锁适合代码少量的同步问题。

小例子:

package com.cn.test.thread.lock;
 
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
 
public class LockTest {
    private Lock lock = new ReentrantLock();
    /*
     * 使用完毕释放后其他线程才能获取锁
     */
    public void lockTest(Thread thread) {
        lock.lock();//获取锁
        try {
            System.out.println("线程"+thread.getName() + "获取当前锁"); //打印当前锁的名称
            Thread.sleep(2000);//为看出执行效果,是线程此处休眠2秒
        } catch (Exception e) {
            System.out.println("线程"+thread.getName() + "发生了异常释放锁");
        }finally {
            System.out.println("线程"+thread.getName() + "执行完毕释放锁");
            lock.unlock(); //释放锁
        }
    }
     
    public static void main(String[] args) {
        LockTest lockTest = new LockTest();
        //声明一个线程 “线程一”
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                lockTest.lockTest(Thread.currentThread());
            }
        }, "thread1");
        //声明一个线程 “线程二”
        Thread thread2 = new Thread(new Runnable() {
 
            @Override
            public void run() {
                lockTest.lockTest(Thread.currentThread());
            }
        }, "thread2");
        // 启动2个线程
        thread2.start();
        thread1.start();
 
    }
}

执行结果:

线程thread2获得执行结果
线程thread2执行完毕释放锁
线程thread1获得当前锁
线程thread1执行完毕释放锁

package com.cn.test.thread.lock;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LockTest {
    private Lock lock = new ReentrantLock();
    
    /*
     * 尝试获取锁 tryLock() 它表示用来尝试获取锁,如果获取成功,则返回true,如果获取失败(即锁已被其他线程获取),则返回false
     */
    public void tryLockTest(Thread thread) {
        if(lock.tryLock()) { //尝试获取锁
            try {
                System.out.println("线程"+thread.getName() + "获取当前锁"); //打印当前锁的名称
                Thread.sleep(2000);//为看出执行效果,是线程此处休眠2秒
            } catch (Exception e) {
                System.out.println("线程"+thread.getName() + "发生了异常释放锁");
            }finally {
                System.out.println("线程"+thread.getName() + "执行完毕释放锁");
                lock.unlock(); //释放锁
            }
        }else{
            System.out.println("我是线程"+Thread.currentThread().getName()+"当前锁被别人占用,我无法获取");
        }
    }
    public static void main(String[] args) {
        LockTest lockTest = new LockTest();
        
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                lockTest.tryLockTest(Thread.currentThread());
            }
        }, "thread1");
        //声明一个线程 “线程二”
        Thread thread2 = new Thread(new Runnable() {

            @Override
            public void run() {
                lockTest.tryLockTest(Thread.currentThread());
            }
        }, "thread2");
        // 启动2个线程
        thread2.start();
        thread1.start();
    }
}

执行结果:

线程thread2获得当前锁
我是线程thread1当前锁被别人占用,我无法获取
线程thread2执行完毕释放锁

package com.cn.test.thread.lock;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LockTest {
    private Lock lock = new ReentrantLock();
    public void tryLockParamTest(Thread thread) throws InterruptedException {
        if(lock.tryLock(3000, TimeUnit.MILLISECONDS)) { //尝试获取锁 获取不到锁,就等3秒,如果3秒后还是获取不到就返回false  
            try {
                System.out.println("线程"+thread.getName() + "获取当前锁"); //打印当前锁的名称
                Thread.sleep(4000);//为看出执行效果,是线程此处休眠2秒
            } catch (Exception e) {
                System.out.println("线程"+thread.getName() + "发生了异常释放锁");
            }finally {
                System.out.println("线程"+thread.getName() + "执行完毕释放锁");
                lock.unlock(); //释放锁
            }
        }else{
            System.out.println("我是线程"+Thread.currentThread().getName()+"当前锁被别人占用,等待3s后仍无法获取,放弃");
        }
    }
    public static void main(String[] args) {
        LockTest lockTest = new LockTest();
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    lockTest.tryLockParamTest(Thread.currentThread());
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }, "thread1");
        //声明一个线程 “线程二”
        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    lockTest.tryLockParamTest(Thread.currentThread());
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }, "thread2");
        // 启动2个线程
        thread2.start();
        thread1.start();
    }
}

执行结果:
因为此时线程1休眠了4秒,线程2等待了3秒还没有获取到就放弃获取锁了,执行结束

线程thread2获得当前锁
我是线程thread1当前锁被别人占用,等待3秒后仍无法获取,放弃
线程thread2执行完毕释放锁

将方法中的 Thread.sleep(4000)改为Thread.sleep(2000)执行结果如下:
因为此时线程1休眠了2秒,线程2等待了3秒的期间线程1释放了锁,此时线程2获取到锁,线程2就可以执行了

线程thread2获得当前锁
线程thread2执行完毕释放锁
线程thread1获得当前锁
线程thread1执行完毕释放锁

转载地址:https://www.cnblogs.com/iyyy/p/7993788.html