实现一个显示锁

Posted 2020-11-23 龙宇在天

定义一个lock接口

package com.dwz.concurrency.chapter10;

import java.util.Collection;

public interface Lock {
    class TimeOutException extends Exception {
        public TimeOutException(String message) {
            super(message);
        }
    }
    
    void lock() throws InterruptedException;
    
    void lock(long mills) throws TimeOutException, InterruptedException;
    
    void unlock();
    
    Collection<Thread> getBlockedThread();
    
    int getBlockedSize();
}

lock的实现类

package com.dwz.concurrency.chapter10;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Optional;
/**
 *      实现一个显示锁lock
 */
public class BooleanLock2 implements Lock {
    //The initValue is true indicated the lock have be get.
    //The initValue is false indicated the lock is free(other thread can get this.)
    private boolean initValue;
    
    private Collection<Thread> blockedThreadCollection = new ArrayList<>();
    
    public BooleanLock2() {
        this.initValue = false;
    }

    @Override
    public synchronized void lock() throws InterruptedException {
        while (initValue) {
            blockedThreadCollection.add(Thread.currentThread());
            Optional.of(Thread.currentThread().getName() + " add the blockedThreadCollection....").ifPresent(System.out::println);
            this.wait();
        }
        
        blockedThreadCollection.remove(Thread.currentThread());
        this.initValue = true;
    }

    @Override
    public synchronized void unlock() {
        this.initValue = false;
        Optional.of(Thread.currentThread().getName() + " release the lock monitor....").ifPresent(System.out::println);
        this.notifyAll();
    }

    @Override
    public Collection<Thread> getBlockedThread() {
        return Collections.unmodifiableCollection(blockedThreadCollection);
    }

    @Override
    public int getBlockedSize() {
        return blockedThreadCollection.size();
    }

    @Override
    public void lock(long mills) throws TimeOutException, InterruptedException {
        return;
    }

}

此处wait()的是this对象，sychronized的也是this对象

测试类

package com.dwz.concurrency.chapter10;

import java.util.Optional;
import java.util.stream.Stream;
/**
 *  多线程的lock
 * @author dangwangzhen
 *
 */
public class LockTest {
    
    private static void work() throws InterruptedException {
        Optional.of(Thread.currentThread().getName() + " is working...").ifPresent(System.out::println);
        Thread.sleep(10_000);
    }
    public static void main(String[] args) throws InterruptedException {
        final BooleanLock2 booleanLock = new BooleanLock2();
        Stream.of("T1", "T2", "T3", "T4")
                .forEach(name -> 
                    new Thread(() -> {
                        try {
                            booleanLock.lock();
                            Optional.of(Thread.currentThread().getName() + " have the lock Monitor...").ifPresent(System.out::println);
                            work();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        } finally {
                            booleanLock.unlock();
                        }
                    }, name).start()
                );
        
        Thread.sleep(100);
        //缺点：该锁可能被其他线程释放(该锁被main线程释放)
        booleanLock.unlock();
    }
}

缺点：如果在线程执行过程中锁被main线程释放，不能保证 线程加的锁被线程自己释放

改进方法

package com.dwz.concurrency.chapter10;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Optional;
/**
 *      实现一个显示锁lock,并保证锁是由自己释放
 */
public class BooleanLock implements Lock {
    //The initValue is true indicated the lock have be get.
    //The initValue is false indicated the lock is free(other thread can get this.)
    private boolean initValue;
    
    //保证该锁只能被当前线程释放
    private Thread currentThread;
    
    private Collection<Thread> blockedThreadCollection = new ArrayList<>();
    
    public BooleanLock() {
        this.initValue = false;
    }

    @Override
    public synchronized void lock() throws InterruptedException {
        while (initValue) {
            blockedThreadCollection.add(Thread.currentThread());
            Optional.of(Thread.currentThread().getName() + " add the blockedThreadCollection....").ifPresent(System.out::println);
            this.wait();
        }
        
        blockedThreadCollection.remove(Thread.currentThread());
        this.initValue = true;
        this.currentThread = Thread.currentThread();
    }

    @Override
    public synchronized void unlock() {
        if(Thread.currentThread() == currentThread) {
            this.initValue = false;
            Optional.of(Thread.currentThread().getName() + " release the lock monitor....").ifPresent(System.out::println);
            this.notifyAll();
        }
    }

    @Override
    public Collection<Thread> getBlockedThread() {
        return Collections.unmodifiableCollection(blockedThreadCollection);
    }

    @Override
    public int getBlockedSize() {
        return blockedThreadCollection.size();
    }

}