ReentrantLock

1.ReentrantLock概述

在java多线程中，可以使用synchronized关键字来实现线程之间同步互斥，但在JDK1.5中增加了ReentrantLock也能达到同样的效果，并且在扩展功能上也更加强大，而且使用上更加灵活。

使用ReentrantLock实现生产者消费者：

package chapter4.reentrant;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class MyStack {
	
	private List<String> list = new ArrayList<>();
	ReentrantLock lock = new ReentrantLock();
	Condition conditionp = lock.newCondition();
	Condition conditionc = lock.newCondition();
	
	public void push() {
		try {
			lock.lock();
			while(list.size() == 1) {
				System.out.println("push 操作中的:"+Thread.currentThread().getName()+" 线程呈等待状态");
				conditionp.await();
			}
			list.add("anything="+Math.random());
			conditionc.signalAll();
			System.out.println("push="+list.size());
		} catch (Exception e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}
	
	public void pop() {
		try {
			lock.lock();
			while(list.size() == 0) {
				System.out.println("pop 操作中的:"+Thread.currentThread().getName()+" 线程呈等待状态");
				conditionc.await();
			}
			String str = list.get(0);
			System.out.println("pop "+str);
			list.remove(0);
			conditionp.signalAll();
			System.out.println("pop="+list.size());
		} catch (Exception e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}
	
}

package chapter4.reentrant;

public class P {
	private MyStack myStack;
	public P(MyStack myStack) {
		this.myStack = myStack;
	}
	
	public void pushService() {
		myStack.push();;
	}

}


package chapter4.reentrant;

public class C {
	private MyStack myStack;
	public C(MyStack myStack) {
		this.myStack = myStack;
	}
	
	public void popService() {
		myStack.pop();;
	}

}


package chapter4.reentrant;

public class ThreadP extends Thread{
	
	private P p;
	public ThreadP(P p) {
		this.p = p;
	}
	
	@Override
	public void run() {
		super.run();
		p.pushService();
	}

}


package chapter4.reentrant;

public class ThreadC extends Thread{
	
	private C c;
	public ThreadC(C c) {
		this.c = c;
	}
	
	@Override
	public void run() {
		super.run();
		c.popService();
	}

}


package chapter4.reentrant;

public class Run {
	
	public static void main(String[] args) {
		try {
			int cnt=3;
			MyStack myStack = new MyStack();
			P[] pArr = new P[cnt];
			C[] cArr = new C[cnt];
			for(int i=0;i<cnt;i++) {
				pArr[i] =new P(myStack); 
				cArr[i] = new C(myStack);;
			}
			ThreadP[] threadPs = new ThreadP[cnt];
			ThreadC[] threadCs = new ThreadC[cnt];
			for(int i=0;i<cnt;i++) {
				threadPs[i] = new ThreadP(pArr[i]);
				threadCs[i] = new ThreadC(cArr[i]);
			}
			for(int i=0;i<cnt;i++) {
				threadCs[i].start();
			}
			Thread.sleep(1000);
			for(int i=0;i<cnt;i++) {
				threadPs[i].start();
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		
	}

}

 运行结果:

pop 操作中的:Thread-1 线程呈等待状态
pop 操作中的:Thread-3 线程呈等待状态
pop 操作中的:Thread-5 线程呈等待状态
push=1
push 操作中的:Thread-2 线程呈等待状态
push 操作中的:Thread-4 线程呈等待状态
pop anything=0.04358985658650283
pop=0
pop 操作中的:Thread-3 线程呈等待状态
pop 操作中的:Thread-5 线程呈等待状态
push=1
push 操作中的:Thread-4 线程呈等待状态
pop anything=0.8139775455313625
pop=0
pop 操作中的:Thread-5 线程呈等待状态
push=1
pop anything=0.8443402579212704
pop=0

2.ReentrantLock方法介绍

•  int getQueueLength():返回正等待获取此锁定的线程估计数。

package chapter4.reentrantmethod;

import java.util.concurrent.locks.ReentrantLock;

public class Service {
	
	public ReentrantLock lock = new ReentrantLock();
	public void method() {
		try {
			lock.lock();
			System.out.println("ThreadName:"+Thread.currentThread().getName()+"进入方法!");
			Thread.sleep(Integer.MAX_VALUE);
		} catch (Exception e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}

}

package chapter4.reentrantmethod;

public class Run {
	
	public static void main(String[] args) {
		try {
			final Service service = new Service();
			Runnable runnable = new Runnable() {
				public void run() {
					service.method();
				}
			};
			Thread[] threads = new Thread[10];
			for(int i=0;i<10;i++) {
				threads[i] = new Thread(runnable);
			}
			for(int i=0;i<10;i++) {
				threads[i].start();
			}
			Thread.sleep(2000);
			System.out.println(service.lock.getQueueLength()+"个线程在等待获取锁");
		} catch (Exception e) {
			e.printStackTrace();
		}
	
	}

}

 运行结果:

ThreadName:Thread-0进入方法!
9个线程在等待获取锁

• int getHoldCount():查询当前线程保持此锁定的个数，也就是调用lock()方法的次数。

package chapter4.reentrantmethod;

import java.util.concurrent.locks.ReentrantLock;

public class Service {
	
	public ReentrantLock lock = new ReentrantLock();
	public void method1() {
		try {
			lock.lock();
			System.out.println("method1 getHoldCount:"+lock.getHoldCount());
			method2();
		} catch (Exception e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}
	
	public void method2() {
		try {
			lock.lock();
			System.out.println("method2 getHoldCount:"+lock.getHoldCount());
		} catch (Exception e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}

}

package chapter4.reentrantmethod;

public class Run {
	
	public static void main(String[] args) {
		Service service = new Service();
		service.method1();
	}

}

 运行结果:

method1 getHoldCount:1
method2 getHoldCount:2

• boolean hasWaiters(Condition condition):查询是否有线程正在等待与此锁定有关的Condition条件。
• int getWaitQueueLength(Condition condition):返回等待与此锁定条件相关的给定条件Condition的线程估计数。

package chapter4.reentrantmethod;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class Service {
	
	private ReentrantLock lock = new ReentrantLock();
	private Condition condition = lock.newCondition();
	public void waitMethod() {
		try {
			lock.lock();
			condition.await();
		} catch (Exception e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}
	
	public void notifyMethod() {
		try {
			lock.lock();
　　　　　　　　　　　　　　System.out.println("是否有线程正在等待condition:"+lock.hasWaiters(condition));
			System.out.println(lock.getWaitQueueLength(condition)+"个线程正在等待condition");
			condition.signalAll();
		} catch (Exception e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}

}

package chapter4.reentrantmethod;

public class Run {
	
	public static void main(String[] args) {
		try {
			final Service service = new Service();
			Runnable runnable = new Runnable() {
				public void run() {
					service.waitMethod();
				}
			};
			Thread[] threads = new Thread[10];
			for(int i=0;i<10;i++) {
				threads[i] = new Thread(runnable);
			}
			for(int i=0;i<10;i++) {
				threads[i].start();
			}
			Thread.sleep(2000);
			service.notifyMethod();
		} catch (Exception e) {
			e.printStackTrace();
		}
	
	}

}

 运行结果:

是否有线程正在等待condition:true
10个线程正在等待condition

•  boolean hasQueuedThread(Thread thread):查询指定的线程是否正在等待获取此锁定。
• boolean hasQueuedThreads():查询是否有线程正在等待获取此锁定。

package chapter4.reentrantmethod;

import java.util.concurrent.locks.ReentrantLock;

public class Service2 {
	
	public ReentrantLock lock = new ReentrantLock();
	public void waitMethod() {
		try {
			lock.lock();
			Thread.sleep(Integer.MAX_VALUE);
		} catch (Exception e) {
			e.printStackTrace();
		}finally {
			lock.unlock();
		}
	}

}

package chapter4.reentrantmethod;

public class Run2 {
	
	public static void main(String[] args) {
		try {
			final Service2 service = new Service2();
			Runnable runnable = new Runnable() {
				public void run() {
					service.waitMethod();
				}
			};
			Thread threada = new Thread(runnable);
			threada.start();
			Thread.sleep(500);
			
			Thread threadb = new Thread(runnable);
			threadb.start();
			Thread.sleep(500);
			System.out.println(service.lock.hasQueuedThread(threada));
			System.out.println(service.lock.hasQueuedThread(threadb));
			System.out.println(service.lock.hasQueuedThreads());
		} catch (Exception e) {
			e.printStackTrace();
		}
	
	}

}

 执行结果:

false
true
true

• boolean isFair():判断是不是公平锁，ReentrantLock类默认使用的是非公平锁。
• boolean isHeldByCurrentThread():查询当前线程是否保持此锁定。
• boolean isLocked():此锁定是否由任意线程保持。
• void lockInterruptibly():如果当前线程未被中断，则获取锁定，如果已经被中断则出现异常。
• boolean tryLock():仅在调用时锁定未被另一个线程保持的情况下，才获取该锁定。
• boolean tryLock(long timeout, TimeUnit unit):如果锁定在给定等待时间内没有被另一个线程保持，且当前线程未被中断，则获取该锁定。
• void awaitUninterruptibly():线程被中断，不会抛出异常。
• boolean awaitUntil(Date deadline):wait到某一个时间点，可以被提前唤醒。

3.ReentrantReadWriteLock

• readLock().lock();//获取读锁
• readLock().unlock();//释放读锁
• writeLock().lock();//获取写锁
• writeLock().unlock();//释放写锁

读读共享，写写互斥，读写互斥，写读互斥。