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java并發(fā)編程中實現(xiàn)可見性的四種可行方案解析

更新時間：2023年08月07日 09:04:53 作者：鍵盤林

這篇文章主要介紹了java并發(fā)編程中實現(xiàn)可見性的四種可行方案解析,使用關(guān)鍵字volatile和使用鎖(如synchronized關(guān)鍵字或者java.util.concurrent包中的鎖)來確保對共享變量的修改在多線程環(huán)境中能夠正確地被其他線程所觀察到,需要的朋友可以參考下

在方法或者變量已經(jīng)同步的情況下,還會出現(xiàn)什么問題嗎?

多線程編程主要學(xué)的就是"互斥"和"可見","互斥"指的就是就是同步,而"可見"想必很多人還沒有理解.

舉個例子:

MyVolatile.java:

package cn.mxl.test.wr;
public class MyVolatile {
	private  int a=0;
	public   void write() throws InterruptedException {
		a++;
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
	public   void read() throws InterruptedException {
		while(a==0) {
//			Thread.sleep(1);
//			System.out.println(a);
		}
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
}

MyRead.java:

package cn.mxl.test.wr;
public class MyRead implements Runnable{
	private MyVolatile task;
	public MyRead(MyVolatile task) {
		// TODO Auto-generated constructor stub
		this.task=task;
	}
	@Override
	public void run() {
		// TODO Auto-generated method stub
		try {
			task.read();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

MyWrite.java:

package cn.mxl.test.wr;
public class MyWrite implements Runnable{
	private MyVolatile task;
	public MyWrite(MyVolatile task) {
		// TODO Auto-generated constructor stub
		this.task=task;
	}
	@Override
	public void run() {
		// TODO Auto-generated method stub
		try {
			task.write();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

Run.java:

package cn.mxl.test.wr;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.ReentrantLock;
public class Run {
	public static void main(String[] args) throws Exception, ExecutionException {
		MyVolatile task=new MyVolatile();
		MyWrite mt1=new MyWrite(task);
		MyRead mt2=new MyRead(task);
		Thread tW=new Thread(mt1,"Write");
		Thread tR=new Thread(mt2,"Read");
		tR.start();
		Thread.sleep(2000);
		tW.start();
	}
}

運行結(jié)果:

Read線程一直處于a==0中是為什么呢？

因為當(dāng)Read線程先開始線程的時候,加載主內(nèi)存中的數(shù)據(jù)放到自己的私有內(nèi)存中,在程序執(zhí)行過程中為了提高效率,它就只讀自己私有內(nèi)存中的數(shù)據(jù),不會再去讀主內(nèi)存中的共享數(shù)據(jù),在這個過程中,主內(nèi)存對于該線程來說,只寫不讀,也就是說將自己修改的變量值存放到主內(nèi)存中,但是自己不去讀主內(nèi)存的數(shù)據(jù);所以說,在Read線程執(zhí)行到while語句一直循環(huán)的時候,即使Write線程修改a==1值了,Read線程還是之前a==0的值,一直跳不出循環(huán);

解決辦法-->可見性,讓Read線程讀a變量的時候去主內(nèi)存(共享數(shù)據(jù))中讀取:

volatiel關(guān)鍵字的使用:

MyVolatile.java:

package cn.mxl.test.wr;
public class MyVolatile {
	private volatile  int a=0;
	public   void write() throws InterruptedException {
		a++;
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
	public   void read() throws InterruptedException {
		while(a==0) {
//			Thread.sleep(1);
//			System.out.println(a);
		}
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
}

結(jié)果:

其實在這個過程中,我遇到了一些有趣的事情,可見性這東西,其實還有三個東西可以實現(xiàn):

第一個-->Thread.sleep():

MyVolatile.java(取消sleep的注解):

package cn.mxl.test.wr;
public class MyVolatile {
	private   int a=0;
	public   void write() throws InterruptedException {
		a++;
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
	public   void read() throws InterruptedException {
		while(a==0) {
			Thread.sleep(1);
//			System.out.println(a);
		}
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
}

結(jié)果:

第二個是system.out.println():

MyVolatile.java:

package cn.mxl.test.wr;
public class MyVolatile {
	private   int a=0;
	public   void write() throws InterruptedException {
		a++;
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
	public   void read() throws InterruptedException {
		while(a==0) {
//			Thread.sleep(1);
			System.out.println(a);
		}
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
}

結(jié)果:

第三個atomic:

MyVolatile.java:

package cn.mxl.test.wr.atomic;
import java.util.concurrent.atomic.AtomicInteger;
public class MyVolatile {
	private   AtomicInteger a=new AtomicInteger();
	public   void write() throws InterruptedException {
		a.incrementAndGet();
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
	public   void read() throws InterruptedException {
		while(a.get()==0) {
//			Thread.sleep(1);
//			System.out.println(a);
		}
		System.out.println("當(dāng)前線程:"+Thread.currentThread().getName());
		System.out.println(a);
	}
}

結(jié)果: