Java 中锁之间的对比

synchronized 和  java.util.concurrent.lock.Lock 之间的区别

• 实现层面不一样。synchronized 是 Java 关键字，JVM层面 实现加锁和释放锁；Lock 是一个接口，在代码层面实现加锁和释放锁
• 是否自动释放锁。synchronized 在线程代码执行完或出现异常时自动释放锁；Lock 不会自动释放锁，需要在 finally {} 代码块显式地中释放锁
• 是否一直等待。synchronized 会导致线程拿不到锁一直等待；Lock 可以设置尝试获取锁或者获取锁失败一定时间超时
• 获取锁成功是否可知。synchronized 无法得知是否获取锁成功；Lock 可以通过 tryLock 获得加锁是否成功
• 功能复杂性。synchronized 加锁可重入、不可中断、非公平；Lock 可重入、可判断、可公平和不公平、细分读写锁提高效率

java.util.concurrent.lock.Lock 与  java.util.concurrent.lock.ReadWriteLock 之间的区别

• ReadWriteLock 定义了获取读锁和写锁的接口，读锁之间不互斥，非常适合读多、写少的场景

适用场景

• JDK 1.6 开始，对 synchronized 方式枷锁进行了优化，加入了偏向锁、轻量级锁和锁升级机制，性能得到了很大的提升。性能与 ReentrantLock  差不多
• 读多写少的情况下，考虑使用 ReadWriteLock 

synchronized、ReentrantLock、ReentrantReadWriteLock 启动 990 个线程读共享变量，10 个线程写共享变量

package constxiong.concurrency.a020;

import java.util.ArrayList;
import java.util.List;
import java.util.UUID;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * JDK 1.8 中锁性能的测试
 * @author ConstXiong
 */
public class TestLockPerformance {
    
    public static Object obj = new Object();//用于 synchronized 获取锁
    
    public static Lock lock = new ReentrantLock();//可重入锁
    
    public static ReadWriteLock readWriteLock = new ReentrantReadWriteLock();//读写锁
    
    public static final int READ = 0;
    
    public static final int WRITE = 1;
    
    // uuid，一个随机字符串
    public static String uuid = UUID.randomUUID().toString();

    public static void main(String[] args) throws InterruptedException {
//        testSynchronized(1000);
        
        testReentrantLock(1000);
        
//        testReadWriteLock(1000);
    }
    
    public static void testSynchronized(int threadNum) throws InterruptedException {
        long t1 = System.currentTimeMillis();
        List<Thread> tList = new ArrayList<Thread>();
        //启动 threadNum - 向上取整 (0.01 * threadNum) 个线程读 uuid, 向上取整 (0.01 * threadNum) 个线程写 uuid
        for (int i = 0; i < threadNum; i++) {
            Thread t;
            if (i % 100 == 0) {
                t = new Thread(new WorkerSynchronized(WRITE));
            } else {
                t = new Thread(new WorkerSynchronized(READ));
            }
            t.start();//启动线程
            tList.add(t);
        }
        
        for (Thread t : tList) {
            t.join();
        }
        
        long t2 = System.currentTimeMillis();
        System.out.println("testSynchronized 耗时：" + (t2 - t1));
    }
    
    public static void testReentrantLock(int threadNum) throws InterruptedException {
        long t1 = System.currentTimeMillis();
        List<Thread> tList = new ArrayList<Thread>();
        //启动 threadNum - 向上取整 (0.01 * threadNum) 个线程读 uuid, 向上取整 (0.01 * threadNum) 个线程写 uuid
        for (int i = 0; i < threadNum; i++) {
            Thread t;
            if (i % 100 == 0) {
                t = new Thread(new WorkerReentrantLock(WRITE));
            } else {
                t = new Thread(new WorkerReentrantLock(READ));
            }
            t.start();//启动线程
            tList.add(t);
        }
        
        for (Thread t : tList) {
            t.join();
        }
        
        long t2 = System.currentTimeMillis();
        System.out.println("testReentrantLock 耗时：" + (t2 - t1));
    }
    
    public static void testReadWriteLock(int threadNUm) throws InterruptedException {
        long t1 = System.currentTimeMillis();
        List<Thread> tList = new ArrayList<Thread>();
        //启动 threadNum - 向上取整 (0.01 * threadNum) 个线程读 uuid, 向上取整 (0.01 * threadNum) 个线程写 uuid
        for (int i = 0; i < threadNUm; i++) {
            Thread t;
            if (i % 100 == 0) {
                t = new Thread(new WorkerReadWriteLock(WRITE));
            } else {
                t = new Thread(new WorkerReadWriteLock(READ));
            }
            t.start();//启动线程
            tList.add(t);
        }
        
        for (Thread t : tList) {
            t.join();
        }
        
        long t2 = System.currentTimeMillis();
        System.out.println("testReadWriteLock 耗时：" + (t2 - t1));
    }
    
}

//工作线程，使用 synchronized 关键字加锁
class WorkerSynchronized implements Runnable {
    //0-read;1-write
    private int type;
    
    WorkerSynchronized(int type) {
        this.type = type;
    }
    
    //加锁读 TestLockPerformance.uuid 变量，并打印
    private void read() {
        synchronized (TestLockPerformance.obj) {
            //休眠 20 毫秒，模拟任务执行耗时
            try {
                Thread.sleep(20);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + 
                    " read uuid = " +  TestLockPerformance.uuid);
        }
    }
    
    //加锁写 TestLockPerformance.uuid 变量，并打印
    private void write() {
        synchronized (TestLockPerformance.obj) {
            //休眠 20 毫秒，模拟任务执行耗时
            try {
                Thread.sleep(20);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            TestLockPerformance.uuid = UUID.randomUUID().toString();
            System.out.println(Thread.currentThread().getName() + 
                    " write uuid = " +  TestLockPerformance.uuid);
        }
    }
    
    @Override
    public void run() {
        //type = 0,线程读 TestLockPerformance.uuid 变量
        if (type == 0) {
            read();
        //type = 1,线程生成 uuid，写入 TestLockPerformance.uuid 变量
        } else {
            write();
        }
    }
}

//工作线程，使用 ReentrantLock 加锁
class WorkerReentrantLock implements Runnable {
    //0-read;1-write
    private int type;
    
    WorkerReentrantLock(int type) {
        this.type = type;
    }
    
    //加锁读 TestLockPerformance.uuid 变量，并打印
    private void read() {
        TestLockPerformance.lock.lock();
        try {
            //休眠 20 毫秒，模拟任务执行耗时
            try {
                Thread.sleep(20);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + 
                    " read uuid = " +  TestLockPerformance.uuid);
        } finally {
            TestLockPerformance.lock.unlock();
        }
        
    }
    
    //加锁写 TestLockPerformance.uuid 变量，并打印
    private void write() {
        TestLockPerformance.lock.lock();
        try {
            //休眠 20 毫秒，模拟任务执行耗时
            try {
                Thread.sleep(20);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            TestLockPerformance.uuid = UUID.randomUUID().toString();
            System.out.println(Thread.currentThread().getName() + 
                    " write uuid = " +  TestLockPerformance.uuid);
        } finally {
            TestLockPerformance.lock.unlock();
        }
    }
    
    @Override
    public void run() {
        //type = 0,线程读 TestLockPerformance.uuid 变量
        if (type == 0) {
            read();
        //type = 1,线程生成 uuid，写入 TestLockPerformance.uuid 变量
        } else {
            write();
        }
    }
}


//工作线程，使用 ReentrantReadWriteLock 关键字加锁
class WorkerReadWriteLock implements Runnable {
    //0-read;1-write
    private int type;
    
    WorkerReadWriteLock(int type) {
        this.type = type;
    }
    
    //加锁读 TestLockPerformance.uuid 变量，并打印
    private void read() {
        TestLockPerformance.readWriteLock.readLock().lock();
        try {
            //休眠 20 毫秒，模拟任务执行耗时
            try {
                Thread.sleep(20);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + 
                    " read uuid = " +  TestLockPerformance.uuid);
        } finally {
            TestLockPerformance.readWriteLock.readLock().unlock();
        }
    }
    
    //加锁写 TestLockPerformance.uuid 变量，并打印
    private void write() {
        TestLockPerformance.readWriteLock.writeLock().lock();
        try {
            //休眠 20 毫秒，模拟任务执行耗时
            try {
                Thread.sleep(20);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            TestLockPerformance.uuid = UUID.randomUUID().toString();
            System.out.println(Thread.currentThread().getName() + 
                    " write uuid = " +  TestLockPerformance.uuid);
        } finally {
            TestLockPerformance.readWriteLock.writeLock().unlock();
        }
    }
    
    @Override
    public void run() {
        //type = 0,线程读 TestLockPerformance.uuid 变量
        if (type == 0) {
            read();
        //type = 1,线程生成 uuid，写入 TestLockPerformance.uuid 变量
        } else {
            write();
        }
    }
}

调用测试方法 

testSynchronized(1000);

耗时

Thread-0 write uuid = b7fb63d7-79cc-4cc0-84ed-5a9cd4de6824
Thread-252 read uuid = b7fb63d7-79cc-4cc0-84ed-5a9cd4de6824
Thread-251 read uuid = b7fb63d7-79cc-4cc0-84ed-5a9cd4de6824
.
.
.
Thread-255 read uuid = d666bfe6-dc71-4df2-882a-d530a59d7e92
Thread-254 read uuid = d666bfe6-dc71-4df2-882a-d530a59d7e92
Thread-253 read uuid = d666bfe6-dc71-4df2-882a-d530a59d7e92
testSynchronized 耗时：22991

调用测试方法 

testReentrantLock(1000);

耗时

Thread-0 write uuid = 4352eb13-d284-47ec-8caa-fc81d91d08e1
Thread-1 read uuid = 4352eb13-d284-47ec-8caa-fc81d91d08e1
Thread-485 read uuid = 4352eb13-d284-47ec-8caa-fc81d91d08e1
.
.
.
Thread-997 read uuid = 9d7f0a78-5eb7-4506-9e98-e8e9a7a717a5
Thread-998 read uuid = 9d7f0a78-5eb7-4506-9e98-e8e9a7a717a5
Thread-999 read uuid = 9d7f0a78-5eb7-4506-9e98-e8e9a7a717a5
testReentrantLock 耗时：22935

调用测试方法 

testReadWriteLock(1000);

耗时

Thread-0 write uuid = 81c13f80-fb19-4b27-9d21-2e99f8c8acbd
Thread-277 read uuid = 81c13f80-fb19-4b27-9d21-2e99f8c8acbd
Thread-278 read uuid = 81c13f80-fb19-4b27-9d21-2e99f8c8acbd
.
.
.
Thread-975 read uuid = 35be0359-1973-4a4f-85b7-918053d841f7
Thread-971 read uuid = 35be0359-1973-4a4f-85b7-918053d841f7
Thread-964 read uuid = 35be0359-1973-4a4f-85b7-918053d841f7
testReadWriteLock 耗时：543

通过耗时测试可以看出，使用 synchronized 和 ReentrantLock 耗时相近；但是由于 990 个线程读，10 个线程写，使用 ReentrantReadWriteLock 耗时 543 毫秒。

PS：JDK 并发包里的工具类，还有很多适合特定场景的工具，后面我们继续探索。

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