java 多线程高并发读写控制 误区

先看一下下面的错误代码，对写加了synchronized控制，保证了写的安全，但是问题在哪里呢？

public class testTh7 {
	private String data;
	public String read(){
		System.out.println(Thread.currentThread().getName() + "read data " + data);
		return this.data;
	}
	public  synchronized void write(String data){
		System.out.println(Thread.currentThread().getName() + " === write data " + data);
		this.data = data;
	}

	public static void main(String[] args) {
		final testTh7 t = new testTh7();
		for (int i = 0; i < 100; i++) {
			if (i % 2 == 0) {
				new Thread(new Runnable() {
					@Override
					public void run() {
						Double d = Math.random();
						t.write(d.toString());
					}
				}).start();
			} else {
				new Thread(new Runnable() {
					@Override
					public void run() {
						t.read();
					}
				}).start();
			}
		}
	}
}

 程序的前部分输出结果如下：

Thread-1read data null

Thread-0 === write data 0.8429852467390618

Thread-2 === write data 0.3111022600208211

Thread-3read data 0.3111022600208211

Thread-4 === write data 0.13391602356879362

Thread-5read data 0.13391602356879362

Thread-8 === write data 0.3014059888796128

Thread-6 === write data 0.7073336550466512

Thread-7read data 0.7073336550466512

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Thread-9read data 0.7073336550466512

Thread-10 === write data 0.3157260014049781

Thread-11read data 0.3157260014049781

Thread-15read data 0.3157260014049781

Thread-14 === write data 0.9981422731405993

Thread-16 === write data 0.9011910270245219

Thread-12 === write data 0.34975057489898076

Thread-13read data 0.34975057489898076

Thread-17read data 0.34975057489898076

Thread-18 === write data 0.19089943846264656

Thread-19read data 0.19089943846264656

Thread-21read data 0.19089943846264656

Thread-20 === write data 0.38498810226852065

Thread-22 === write data 0.29234432278529343

Thread-23read data 0.29234432278529343

Thread-27read data 0.29234432278529343

Thread-24 === write data 0.28981062022967496

Thread-29read data 0.28981062022967496

Thread-28 === write data 0.1022791336198855

Thread-26 === write data 0.15466728987586276

Thread-25read data 0.15466728987586276

Thread-31read data 0.15466728987586276

Thread-32 === write data 0.13431233603464776

Thread-33read data 0.13431233603464776

Thread-35read data 0.13431233603464776

Thread-38 === write data 0.33289010029186195

Thread-37read data 0.13431233603464776

Thread-34 === write data 0.5545937895404677

Thread-30 === write data 0.9567137584265717

Thread-36 === write data 0.6461050880921616

可以看到Thread-37这行读取的数据已经不正确了，读取的上一次的旧数据，这不是线程安全的缓存设计。首先确认一下原则，只能有一个线程写操作，可以多个线程并发读，但是写时不能读，读时候不能写。

解决办法有多种，可以使用信号量，使用两个信号量，一个是读写的信号量，还有个是写的信号量，如果简单一点，也可以使用java已经封装的读写锁，代码如下，可以自己测试。

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class testReadWrite {

	private String data = null;
	private ReentrantReadWriteLock rw = new ReentrantReadWriteLock();

	public void read() {
		rw.readLock().lock();
		System.out.println(Thread.currentThread().getName() + " read data!");
		System.out.println(Thread.currentThread().getName() + "read data " + data);
		rw.readLock().unlock();
	}

	public void write(String data) {
		rw.writeLock().lock();
		System.out.println(Thread.currentThread().getName() + " ===write data!");
		this.data = data;
		System.out.println(Thread.currentThread().getName() + " ===write data: " + data);
		rw.writeLock().unlock();
	}

}