Java并发编程——自定义Lock锁

左手的ㄟ右手 2022-03-09 10:06 582阅读 0赞

## 一、引言 ##

在学习JUC并发包的时，会介绍Lock锁。为了更深入的了解Lock锁的机制，我们可以自定义一个Lock锁，这样去读Lock源码可能好理解一点。

## 二、自定义Lock锁 ##

###### 1、定义Lock接口 ######

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

###### 2、Lock实现类 ######

public class BooleanLock implements Lock { 
    
    	// 表示对象的锁是否被占用，true表示对象锁已经被占用
    	private boolean initValue;
    
    	// 表示被阻塞线程的集合
    	private Collection<Thread> blockedThreadCollection = new ArrayList<>();
    
    	// 记录获得当前对象锁的线程
    	private Thread currentThread;
    
    	public BooleanLock() { 
    		this.initValue = false;
    	}
    
    	/** * 加锁，使用synchronized实现同步 */
    	@Override
    	public synchronized void lock() throws InterruptedException { 
    		// 如果锁已经被占用，则阻塞当前线程
    		while (initValue) { 
    			//将线程加入到阻塞线程集合
    			blockedThreadCollection.add(Thread.currentThread());
    			this.wait();
    		}
    
    		// 锁没有被占用，则当前线程获得锁
    		blockedThreadCollection.remove(Thread.currentThread());
    		this.initValue = true;
    		this.currentThread = Thread.currentThread();
    	}
    
    	/** * 当线程等待一定时间后，没有释放锁，则其他线程抛出超时异常 */
    	@Override
    	public synchronized void lock(long mills) throws InterruptedException, TimeOutException { 
    		if (mills <= 0)
    			lock();
    
    		long hasRemaining = mills;
    		long endTime = System.currentTimeMillis() + mills;
    		while (initValue) { 
    			if (hasRemaining <= 0)
    				throw new TimeOutException("Time out");
    			blockedThreadCollection.add(Thread.currentThread());
    			this.wait(mills);
    			hasRemaining = endTime - System.currentTimeMillis();
    		}
    
    		this.initValue = true;
    		this.currentThread = Thread.currentThread();
    	}
    
    	/** * 释放锁 */
    	@Override
    	public synchronized void unlock() { 
    		// 只有占用锁对象的线程，才能释放锁。不然main线程释放锁，程序会有问题。
    		if (Thread.currentThread() == currentThread) { 
    			this.initValue = false;
    			System.out.println(Thread.currentThread().getName() + " release the lock monitor.");
    			this.notifyAll();
    		}
    	}
    
    	@Override
    	public Collection<Thread> getBlockedThread() { 
    		//保证更新集合操作安全，不存在安全问题
    		return Collections.unmodifiableCollection(blockedThreadCollection);
    	}
    
    	@Override
    	public int getBlockedSize() { 
    		return blockedThreadCollection.size();
    	}
    }

###### 3、测试 ######

public class LockTest { 
    
    	public static void main(String[] args) throws InterruptedException { 
    
    		final BooleanLock booleanLock = new BooleanLock();
    
    		//创建4个线程
    		for (int i = 1; i < 5; i++) { 
    
    			new Thread("T" + i) { 
    				@Override
    				public void run() { 
    					try { 
    						booleanLock.lock(10000);
    						System.out.println(Thread.currentThread().getName() + " have the lock Monitor");
    						work();
    					} catch (InterruptedException e) { 
    						e.printStackTrace();
    					} catch (Lock.TimeOutException e) { 
    						System.out.println(Thread.currentThread().getName() + " time out");
    					} finally { 
    						booleanLock.unlock();
    					}
    				};
    			}.start();
    		}
    
    	}
    
    	private static void work() throws InterruptedException { 
    		System.out.println(Thread.currentThread().getName() + " is Working...");
    		Thread.sleep(40_000);
    	}
    }

###### 4、测试结果 ######

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3p4ZDE0MzU1MTM3NzU_size_16_color_FFFFFF_t_70]

## 三、小结 ##

自定义一个Lock锁，熟悉Synchronized的同时，深入理解JUC中的Lock。

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