Java优雅的实现锁和超时锁

左手的ㄟ右手 2021-12-11 06:21 355阅读 0赞

在Java多线程中，我们会遇到多个线程访问同一个资源，产生竞争，如果操作不到会导致死锁的产生，例如在现实中的十字路口，锁就像红路灯指示器，一旦锁坏了，就会导致交通瘫痪。

通过本篇文章读者可以学到以下内容

1.  多线程并发
2.  锁的使用
3.  超时锁

### 一、编写锁的接口 ###

public interface Lock { 
    	//自定义异常类
        public static class TimeOutException extends Exception{ 
            public TimeOutException(String message){ 
                super(message);
            }
        }
    	//无超时锁，可以被打断
        void lock() throws  InterruptedException;
        //超时锁，可以被打断
        void lock(long molls) throws InterruptedException,TimeOutException;
        //解锁
        void unlock();
        //获取当前等待的线程
        Collection<Thread> getBlockedThread();
        //获取当前阻塞的线程数目
        int getBlockSize();
    }

### 二、实现接口，编写功能 ###

首先介绍几个重要的方法属性  
1，Monitor这里选择的是`BooleanLock` ，使用`this.wait()`来阻塞,  
2，因为是多线程访问，因此使用Collection来存储等待的线程  
3，使用while(initValue),通过标志位initValue来判断当前线程是否在工作中，以进行后面的操作  
4，所谓超时锁就是设置超时时间，如果超过这读段时间，抛出异常

public class BooleanLock  implements Lock{ 
        private boolean initValue;
        private Thread currenThread;
        public BooleanLock(){ 
            this.initValue = false;
        }
        private Collection<Thread> blockThreadCollection = new ArrayList<>();
    
        @Override
        public synchronized void lock() throws InterruptedException { 
            while (initValue){ 
                blockThreadCollection.add(Thread.currentThread());
                this.wait();
            }
            //表明此时正在用，别人进来就要锁住
            this.initValue = true;
            currenThread = Thread.currentThread();
            blockThreadCollection.remove(Thread.currentThread());//从集合中删除
        }
    
        @Override
        public synchronized void lock(long mills) throws InterruptedException, TimeOutException { 
            if (mills<=0){ 
                lock();
            }else { 
                long hasRemain = mills;
                long endTime = System.currentTimeMillis()+mills;
                while (initValue){ 
                    if (hasRemain<=0)
                        throw new TimeOutException("Time out");
                    blockThreadCollection.add(Thread.currentThread());
                    hasRemain = endTime-System.currentTimeMillis();
                }
                this.initValue = true;
                currenThread = Thread.currentThread();
            }
    
    
        }
    
        @Override
        public synchronized void unlock() { 
            if (currenThread==Thread.currentThread()){ 
                this.initValue = false; //表明锁已经释放
                Optional.of(Thread.currentThread().getName()+ " release the lock monitor").ifPresent(System.out::println);
                this.notifyAll();
            }
        }
    
        @Override
        public Collection<Thread> getBlockedThread() { 
            return Collections.unmodifiableCollection(blockThreadCollection);
        }
    
        @Override
        public int getBlockSize() { 
            return blockThreadCollection.size();
        }
    }

### 三、测试 ###

public class BlockTest { 
        public static void main(String[] args) throws InterruptedException { 
        final BooleanLock booleanLock = new BooleanLock();
        // 使用Stream流的方式房间四个线程
            Stream.of("T1","T2","T3","T4").forEach(name->{ 
                new Thread(()->{ 
                    try { 
                        booleanLock.lock(10);
                        Optional.of(Thread.currentThread().getName()+" have the lock Monitor").ifPresent(System.out::println);
                        work();
                    } catch (InterruptedException e) { 
                        e.printStackTrace();
                    } catch (Lock.TimeOutException e) { 
                        Optional.of(Thread.currentThread().getName()+" time out").ifPresent(System.out::println);
                    } finally { 
                        booleanLock.unlock();
                    }
                },name).start();
            });
        }
    
        private static void work() throws InterruptedException{ 
            Optional.of(Thread.currentThread().getName()+" is working.....'").ifPresent(System.out::println);
            Thread.sleep(40_000);
        }
    }

结果：

T1 have the lock Monitor
    T1 is working.....'
    T2 time out
    T4 time out
    T3 time out