在Java代码中经常见到synchronized这个关键字。
许多老鸟也经常使用synchronized。但是，你确定知道synchronized怎么用吗？
本文简要介绍一下synchronized的用法之作用于语句块，锁定静态实例对象
前期准备工作（包括相关工具或所使用的原料等）javasynchronizedThreadIntelliJ IDEA 详细的操作方法或具体步骤
synchronized使用于一个语句块，锁定一个静态对象的实例，是类级别的锁，
同一个对象在这个语句块一次只能由一个线程执行。
Talk is cheap.Show me the code.
package chapter2;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.concurrent.TimeUnit;
/**
* Created by MyWorld on 2016/3/14.
*/
public class SynchronizedDemo {
public static void main(String[] args) {
Business business=new Business();
Worker workerFirst=new Worker(business);
Worker workerSecond=new Worker(business);
Thread threadFirst=new Thread(workerFirst, "First");
Thread threadSecond=new Thread(workerSecond, "Second"); threadFirst.start();
threadSecond.start();
}
}
class Worker implements Runnable {
private Business business;
public Worker(Business business) {
this.business=business;
}
@Override
public void run() {
try {
business.doBusiness();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class Business {
private static Logger logger=LoggerFactory.getLogger(Business.class); private static final byte[] lock=new byte[0];
public void doBusiness() throws InterruptedException {
synchronized (lock) {
logger.info("enter synchronized monitor");
TimeUnit.SECONDS.sleep(10);
logger.info("leave synchronized monitor");
}
}
}
执行上面的代码，看看两个线程是不是串行执行的呢？是的
Output:
2016-03-15 23:26:35,791 [First] INFO - enter synchronized monitor
2016-03-15 23:26:45,828 [First] INFO - leave synchronized monitor
2016-03-15 23:26:45,828 [Second] INFO - enter synchronized monitor
2016-03-15 23:26:55,828 [Second] INFO - leave synchronized monitor
如果两个Worker对象呢，是不是这样呢？
Code:
Business business=new Business();
Worker workerFirst=new Worker(business);
Worker workerSecond=new Worker(business);
Thread threadFirst=new Thread(workerFirst, "First");
Thread threadSecond=new Thread(workerSecond, "Second");
threadFirst.start();
threadSecond.start();
执行最新的代码，看看打印的执行结果。
仍然是串行执行，类级别的锁嘛。
再说只是两个线程对象，Business对象的实例中有一个
Output:
2016-03-15 23:31:56,312 [First] INFO - enter synchronized monitor
2016-03-15 23:32:06,315 [First] INFO - leave synchronized monitor
2016-03-15 23:32:06,315 [Second] INFO - enter synchronized monitor
2016-03-15 23:32:16,317 [Second] INFO - leave synchronized monitor
上面有篇文章有个例子，不是因为使用两个Business实例，就让两个线程由串行改为并行。synchronized锁定对象的实例是静态时，是不是也会由串行变为并行呢？
Let's have a try!
Code:
Business businessFirst=new Business();
Business businessSecond=new Business();
Worker workerFirst=new Worker(businessFirst);
Worker workerSecond=new Worker(businessSecond);
Thread threadFirst=new Thread(workerFirst, "First");
Thread threadSecond=new Thread(workerSecond, "Second");
threadFirst.start();
threadSecond.start();
执行最新的代码。看看打印的日志，线程有没有并行执行。
是的，没有。因为这个锁加在类级别，JVM中第个类的定义是唯一的嘛，
肯定是串行的了。。
Output:
2016-03-15 23:41:40,708 [First] INFO - enter synchronized monitor
2016-03-15 23:41:50,710 [First] INFO - leave synchronized monitor
2016-03-15 23:41:50,710 [Second] INFO - enter synchronized monitor
2016-03-15 23:42:00,711 [Second] INFO - leave synchronized monitor
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