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java并發(fā)編程中實(shí)現(xiàn)可見(jiàn)性的四種可行方案解析

更新時(shí)間：2023年08月07日 09:04:53 作者：鍵盤(pán)林

這篇文章主要介紹了java并發(fā)編程中實(shí)現(xiàn)可見(jiàn)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(lái)確保對(duì)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(huì)出現(xiàn)什么問(wèn)題嗎?

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

舉個(gè)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(yùn)行結(jié)果:

Read線程一直處于a==0中是為什么呢？

因?yàn)楫?dāng)Read線程先開(kāi)始線程的時(shí)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(guò)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(huì)再去讀主內(nèi)存中的共享數(shù)據(jù),在這個(gè)過(guò)程中,主內(nèi)存對(duì)于該線程來(lái)說(shuō),只寫(xiě)不讀,也就是說(shuō)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(shuō),在Read線程執(zhí)行到while語(yǔ)句一直循環(huán)的時(shí)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(jiàn)性,讓Read線程讀a變量的時(shí)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(shí)在這個(gè)過(guò)程中,我遇到了一些有趣的事情,可見(jiàn)性這東西,其實(shí)還有三個(gè)東西可以實(shí)現(xiàn):

第一個(gè)-->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(gè)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(gè)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:

到此這篇關(guān)于java并發(fā)編程中實(shí)現(xiàn)可見(jiàn)性的四種可行方案解析的文章就介紹到這了,更多相關(guān)java實(shí)現(xiàn)可見(jiàn)性內(nèi)容請(qǐng)搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！

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