ReentrantLock(可重入锁)和synchronized区别

• 可重入性
• 锁的实现（ReentrantLock是JDK实现，synchronized是依赖于JVM实现，前者可以阅读源码个人实现）
• 性能的区别（目前差不多，倾向于synchronized）
• 功能的区别

function.png

当需要使用ReentrantLock的独有功能时使用该锁

package io.haitaoc.concurrency.example.lock;



import io.haitaoc.concurrency.annotation.ThreadSafe;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;


@ThreadSafe
public class LockExample2 {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    public static int count = 0;

    private final static Lock lock = new ReentrantLock();

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    add();
                    semaphore.release();
                } catch (Exception e) {
                    e.printStackTrace();
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        System.out.println("count: "+ count);
    }

    private static void add() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock();
        }
    }
}

package io.haitaoc.concurrency.example.lock;



import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;


/**
 * 悲观读取，执行写入操作坚决不允许进行读取操作
 * 当读多写少时，就会使写操作进入饥饿，一直进入阻塞状态
 */
public class LockExample3 {

    private final Map<String, Data> map = new TreeMap<>();

    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    private final Lock readLock = lock.readLock();

    private final Lock writeLock = lock.writeLock();

    public Data get(String key) {
        readLock.lock();
        try {
            return map.get(key);
        } finally {
            readLock.unlock();
        }
    }

    public Set<String> getAllKeys() {
        readLock.lock();
        try {
            return map.keySet();
        } finally {
            readLock.unlock();
        }
    }

    public Data put(String key, Data value) {
        writeLock.lock();
        try {
            return map.put(key, value);
        } finally {
            readLock.unlock();
        }
    }

    class Data {

    }
}