WriteLock ReadLock

读写锁：分为读锁和写锁，多个读锁不互斥，读锁与写锁互斥，这是由jvm自己控制的，你只要上好相应的锁即可。如果你的代码只读数据，可以很多人同时读，但不能同时写，那就上读锁；如果你的代码修改数据，只能有一个人在写，且不能同时读取，那就上写锁。总之，读的时候上读锁，写的时候上写锁！

ReentrantReadWriteLock会使用两把锁来解决问题，一个读锁，一个写锁

线程进入读锁的前提条件：没有其他线程的写锁

package cn.itcast.lesson12;

import java.io.ObjectOutputStream.PutField;
import java.util.Random;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

import org.omg.CORBA.PUBLIC_MEMBER;

public class ReadWriteLockTest {

 public static void main(String[] args) {
  final Queue3 q3 = new Queue3();

  for (int i=0; i < 3; i++) {
   new Thread() {
    public void run() {
     while (true) {
      q3.get();
     }
    }
   }.start();

   new Thread() {
    public void run() {
     while (true) {
      //传入一个data值
      q3.put(new Random().nextInt(10000));
     }
    }
   }.start();
  }
 }
}
class Queue3{
 
 private Object data = null;//共享數據，只能有一个线程能写该数据，但有多个线程能读该数据
 private ReentrantReadWriteLock rw1 = new ReentrantReadWriteLock();
 
 public void get(){ 
  rw1.readLock().lock();
   System.out.println(Thread.currentThread().getName()+" be ready to read data!");
   try{
    Thread.sleep((long) (Math.random()*1000));
   }catch (Exception e) {
    e.printStackTrace();
   }
   System.out.println(Thread.currentThread().getName()+"have read data: " + data);
  rw1.readLock().unlock(); 
 } 
 
 public void put(Object data){ 
  rw1.writeLock().lock();
   System.out.println(Thread.currentThread().getName()+" be ready to write data!");
   try{
    Thread.sleep((long) (Math.random()*1000));
   }catch (Exception e) {
    e.printStackTrace();
   }
   this.data = data;
   System.out.println(Thread.currentThread().getName()+"have write data: "+ data);
  rw1.writeLock().unlock();
 }
}

打印

Thread-0 be ready to read data!
Thread-2 be ready to read data!
Thread-0have read data: null
Thread-2have read data: null
Thread-1 be ready to write data!
Thread-1have write data: 8157
Thread-3 be ready to write data!
Thread-3have write data: 5388
Thread-3 be ready to write data!
Thread-3have write data: 3672
Thread-3 be ready to write data!
Thread-3have write data: 5668
Thread-3 be ready to write data!
Thread-3have write data: 3150
Thread-3 be ready to write data!
Thread-3have write data: 4848
Thread-3 be ready to write data!
Thread-3have write data: 8036
Thread-3 be ready to write data!
Thread-3have write data: 3333
Thread-3 be ready to write data!
Thread-3have write data: 5873
Thread-3 be ready to write data!
Thread-3have write data: 5840
Thread-3 be ready to write data!