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java多线程基本概述（三）——同步块

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09/30
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1.1、synchronized方法的弊端

package commonutils;public class CommonUtils { public static long beginTime1;
public static long endTime1; public static long beginTime2;
public static long endTime2;
}=============================
package mytask;import commonutils.CommonUtils;public class Task { private String getData1;
private String getData2; public synchronized void doLongTimeTask() {
try {
System.out.println("begin task");
Thread.sleep(3000);
getData1 = "长时间处理任务后从远程返回的值1 threadName="
+ Thread.currentThread().getName();
getData2 = "长时间处理任务后从远程返回的值2 threadName="
+ Thread.currentThread().getName();
System.out.println(getData1);
System.out.println(getData2);
System.out.println("end task");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
=======================================
package mythread;import commonutils.CommonUtils;import mytask.Task;public class MyThread1 extends Thread { private Task task; public MyThread1(Task task) {
super();
this.task = task;
} @Override
public void run() {
super.run();
CommonUtils.beginTime1 = System.currentTimeMillis();
task.doLongTimeTask();
CommonUtils.endTime1 = System.currentTimeMillis();
}}
=============================================
package mythread;import commonutils.CommonUtils;import mytask.Task;public class MyThread2 extends Thread { private Task task; public MyThread2(Task task) {
super();
this.task = task;
} @Override
public void run() {
super.run();
CommonUtils.beginTime2 = System.currentTimeMillis();
task.doLongTimeTask();
CommonUtils.endTime2 = System.currentTimeMillis();
}}

package test;import mytask.Task;
import mythread.MyThread1;
import mythread.MyThread2;import commonutils.CommonUtils;public class Run { public static void main(String[] args) {
Task task = new Task(); MyThread1 thread1 = new MyThread1(task);
thread1.start(); MyThread2 thread2 = new MyThread2(task);
thread2.start(); try {
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
} long beginTime = CommonUtils.beginTime1;
if (CommonUtils.beginTime2 < CommonUtils.beginTime1) {
beginTime = CommonUtils.beginTime2;
} long endTime = CommonUtils.endTime1;
if (CommonUtils.endTime2 > CommonUtils.endTime1) {
endTime = CommonUtils.endTime2;
} System.out.println("耗时：" + ((endTime - beginTime) / 1000));
}
}

输出结果：

begin task
长时间处理任务后从远程返回的值1 threadName=Thread-0
长时间处理任务后从远程返回的值2 threadName=Thread-0
end task
begin task
长时间处理任务后从远程返回的值1 threadName=Thread-1
长时间处理任务后从远程返回的值2 threadName=Thread-1
end task
耗时：6

当把同步方法改为同步代码块时，

package mytask;import commonutils.CommonUtils;public class Task { private String getData1;
private String getData2; public void doLongTimeTask() {
synchronized(Task.class){
try {
System.out.println("begin task");
Thread.sleep(3000);
getData1 = "长时间处理任务后从远程返回的值1 threadName="
+ Thread.currentThread().getName();
getData2 = "长时间处理任务后从远程返回的值2 threadName="
+ Thread.currentThread().getName();
System.out.println(getData1);
System.out.println(getData2);
System.out.println("end task");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} }
}

输出结果：

begin task
长时间处理任务后从远程返回的值1 threadName=Thread-0
长时间处理任务后从远程返回的值2 threadName=Thread-0
end task
begin task
长时间处理任务后从远程返回的值1 threadName=Thread-1
长时间处理任务后从远程返回的值2 threadName=Thread-1
end task
耗时：6

可见，并没有提升效率，这是因为锁定的范围比较广，所以效果和锁方法的差别并不是太。那么可以缩小边界区，也就是资源真正开始竞争的地方。因为类中的成员变量才是资源的竞争对象，所以需要在访问这些变量的地方进行锁定。那么代码改为如下：

package mytask;import commonutils.CommonUtils;public class Task { private String getData1;
private String getData2; public void doLongTimeTask() { try {
System.out.println("begin task");
Thread.sleep(3000);
getData1 = "长时间处理任务后从远程返回的值1 threadName="
+ Thread.currentThread().getName();
getData2 = "长时间处理任务后从远程返回的值2 threadName="
+ Thread.currentThread().getName();
synchronized(Task.class){
System.out.println(getData1);
System.out.println(getData2);
}
System.out.println("end task");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

输出结果：

begin task
begin task
长时间处理任务后从远程返回的值1 threadName=Thread-0
长时间处理任务后从远程返回的值2 threadName=Thread-0
end task
长时间处理任务后从远程返回的值1 threadName=Thread-1
长时间处理任务后从远程返回的值2 threadName=Thread-1
end task
耗时：3

这时候可以看到时间已经减小了，这就出现一部分同步，一部分异步了。如何验证是真的一半同步一半异步呢？

package mytask;public class Task { public void doLongTimeTask() {
for (int i = 0; i < 100; i++) {
System.out.println("nosynchronized threadName="
+ Thread.currentThread().getName() + " i=" + (i + 1));
}
System.out.println("");
synchronized (this) {
for (int i = 0; i < 100; i++) {
System.out.println("synchronized threadName="
+ Thread.currentThread().getName() + " i=" + (i + 1));
}
} }
}

输出结果：

=================================非同步块异步执行
nosynchronized threadName=Thread-0 i=1
nosynchronized threadName=Thread-1 i=1
nosynchronized threadName=Thread-0 i=2
nosynchronized threadName=Thread-1 i=2
nosynchronized threadName=Thread-0 i=3
nosynchronized threadName=Thread-1 i=3
nosynchronized threadName=Thread-0 i=4
nosynchronized threadName=Thread-1 i=4
nosynchronized threadName=Thread-0 i=5
nosynchronized threadName=Thread-1 i=5
nosynchronized threadName=Thread-0 i=6
nosynchronized threadName=Thread-1 i=6
nosynchronized threadName=Thread-0 i=7
nosynchronized threadName=Thread-1 i=7
nosynchronized threadName=Thread-0 i=8
nosynchronized threadName=Thread-1 i=8
=================================同步块同步执行
synchronized threadName=Thread-1 i=85
synchronized threadName=Thread-1 i=86
synchronized threadName=Thread-1 i=87
synchronized threadName=Thread-1 i=88
synchronized threadName=Thread-1 i=89
synchronized threadName=Thread-1 i=90
synchronized threadName=Thread-1 i=91
synchronized threadName=Thread-1 i=92
synchronized threadName=Thread-1 i=93
synchronized threadName=Thread-1 i=94
synchronized threadName=Thread-1 i=95
synchronized threadName=Thread-1 i=96
synchronized threadName=Thread-1 i=97
synchronized threadName=Thread-1 i=98
synchronized threadName=Thread-1 i=99
synchronized threadName=Thread-1 i=100
synchronized threadName=Thread-0 i=1
synchronized threadName=Thread-0 i=2
synchronized threadName=Thread-0 i=3
synchronized threadName=Thread-0 i=4
synchronized threadName=Thread-0 i=5
synchronized threadName=Thread-0 i=6
synchronized threadName=Thread-0 i=7
synchronized threadName=Thread-0 i=8
synchronized threadName=Thread-0 i=9
synchronized threadName=Thread-0 i=10
synchronized threadName=Thread-0 i=11
synchronized threadName=Thread-0 i=12
synchronized threadName=Thread-0 i=13
synchronized threadName=Thread-0 i=14
synchronized threadName=Thread-0 i=15
synchronized threadName=Thread-0 i=16