[图解Java]读写锁ReentrantReadWriteLock

Posted 2020-12-26 GoldArowana &

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图解ReentrantReadWriteLock

如果之前使用过读写锁, 那么可以直接看本篇文章. 如果之前未使用过, 那么请配合我的另一篇文章一起看: [源码分析]读写锁ReentrantReadWriteLock

0. demo

我先给出一个demo, 这样大家就可以根据我给的这段代码, 边调试边看源码了. 还是那句话: 注意"My" , 我把ReentrantReadWriteLock类改名为了 "MyReentrantReadWriteLock"类 , "Lock"类改名为了"MyLock"类. 大家粘贴我的代码的时候, 把相应的"My"都去掉就好了, 否则会编译报错哦.

demo里是一个公平读写锁

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.concurrent.locks.Lock;
import java.util.function.Supplier;

public class ReentrantReadWriteLockTest2 {
    static final Scanner scanner = new Scanner(System.in);
    static volatile String cmd = "";
    private static MyReentrantReadWriteLock lock = new MyReentrantReadWriteLock(true);
    private static MyReentrantReadWriteLock.ReadLock readLock = lock.readLock();
    private static MyReentrantReadWriteLock.WriteLock writeLock = lock.writeLock();

    public static void main(String[] args) {
        for (Map.Entry<String, Lock> entry : new HashMap<String, Lock>() {{
            for (int i = 0; i < 10; i++) {
                put("r" + i, readLock);
                put("w" + i, writeLock);
            }
        }}.entrySet()) {
            new Thread(() -> func(entry::getValue, entry.getKey())).start();
        }

        while (scanner.hasNext()) {
            cmd = scanner.nextLine();
        }
    }

    public static void func(Supplier<Lock> myLockSupplier, String name) {
        String en_type = myLockSupplier.get().getClass().getSimpleName().toLowerCase().split("lock")[0];
        String zn_type = (en_type.equals("read") ? "读" : "写");
        blockUntilEquals(() -> cmd, "lock " + en_type + " " + name);
        myLockSupplier.get().lock();
        System.out.println(name + "获取了" + zn_type + "锁");
        blockUntilEquals(() -> cmd, "unlock " + en_type + " " + name);
        myLockSupplier.get().unlock();
        System.out.println(name + "释放了" + zn_type + "锁");
    }

    private static void blockUntilEquals(Supplier<String> cmdSupplier, final String expect) {
        while (!cmdSupplier.get().equals(expect))
            quietSleep(1000);
    }

    private static void quietSleep(int mills) {
        try {
            Thread.sleep(mills);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

使用例子在下面.

我们可以看到r1持有了读锁之后, r2来申请读锁, 也可以成功. 说明读锁是可以共享的.

接下来申请写锁. 申请的写锁会进入到`等待队列`.

然后咱们再申请读锁r3, r4. 由于咱们的demo是公平的读写锁. `等待队列`中有线程在等待写锁时, 后续的申请读锁的线程也都会直接进入`等待队列`. 不会和先来的写锁线程争抢.

大致就是这样.更详细的请看下面小节的详解.