[圖解Java]讀寫鎖ReentrantReadWriteLock

時間 2019-12-12

標籤圖解 java 讀寫 reentrantreadwritelock 欄目 Java 简体版

原文原文鏈接

圖解ReentrantReadWriteLock

若是以前使用過讀寫鎖, 那麼能夠直接看本篇文章. 若是以前未使用過, 那麼請配合個人另外一篇文章一塊兒看: [源碼分析]讀寫鎖ReentrantReadWriteLockhtml

0. demo

我先給出一個demo, 這樣你們就能夠根據我給的這段代碼, 邊調試邊看源碼了. 仍是那句話: 注意"My" , 我把ReentrantReadWriteLock類更名爲了 "MyReentrantReadWriteLock"類 , "Lock"類更名爲了"MyLock"類. 你們粘貼個人代碼的時候, 把相應的"My"都去掉就行了, 不然會編譯報錯哦.java

demo裏是一個公平讀寫鎖多線程

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") ? "讀" : "寫");
        blockUntilEquals(() -> cmd, "lock " + en_type + " " + name);
        myLockSupplier.get().lock();
        System.out.println(name + "獲取了" + zn_type + "鎖");
        blockUntilEquals(() -> cmd, "unlock " + en_type + " " + name);
        myLockSupplier.get().unlock();
        System.out.println(name + "釋放了" + zn_type + "鎖");
    }

    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. 源碼分析

咱們能夠看到r1持有了讀鎖以後, r2來申請讀鎖, 也能夠成功. 說明讀鎖是能夠共享的.ui

接下來申請寫鎖. 申請的寫鎖會進入到`等待隊列`.spa

而後我們再申請讀鎖r3, r4. 因爲我們的demo是公平的讀寫鎖. `等待隊列`中有線程在等待寫鎖時, 後續的申請讀鎖的線程也都會直接進入`等待隊列`. 不會和先來的寫鎖線程爭搶.線程

大體就是這樣.更詳細的請看下面小節的詳解.3d