1.2.4注意Sysyem.out.println与i--

 package com.cky.thread;

 /**
  * Created by chenkaiyang on 2017/11/27.
  */
 public class MyThreadThird  extends Thread{
     private int count = 5;
     @Override
     public void run() {
         super.run();
         System.out.println("由" + this.currentThread().getName()+"计算count="+count--);
     }
 }

 package com.cky.test;

 import com.cky.thread.MyThreadThird;

 /**
  * Created by chenkaiyang on 2017/11/27.
  */
 public class Test3 {
     public static void main(String[] args) {
         MyThreadThird mythread = new MyThreadThird();
         Thread a = new Thread(mythread, "A");
         Thread b = new Thread(mythread, "B");
         Thread c = new Thread(mythread, "C");
         Thread d = new Thread(mythread, "D");
         Thread e = new Thread(mythread, "E");

         a.start();
         b.start();
         c.start();
         d.start();
         e.start();

     }
 }

结果如下

D:\it\jdk1.8\bin\java -Didea.launcher.port=7540 "-Didea.launcher.bin.path=D:\it\idea\IntelliJ IDEA 2016.3.3\bin" -Dfile.encoding=UTF-8 -classpath "D:\it\jdk1.8\jre\lib\charsets.jar;D:\it\jdk1.8\jre\lib\deploy.jar;D:\it\jdk1.8\jre\lib\ext\access-bridge-64.jar;D:\it\jdk1.8\jre\lib\ext\cldrdata.jar;D:\it\jdk1.8\jre\lib\ext\dnsns.jar;D:\it\jdk1.8\jre\lib\ext\jaccess.jar;D:\it\jdk1.8\jre\lib\ext\jfxrt.jar;D:\it\jdk1.8\jre\lib\ext\localedata.jar;D:\it\jdk1.8\jre\lib\ext\nashorn.jar;D:\it\jdk1.8\jre\lib\ext\sunec.jar;D:\it\jdk1.8\jre\lib\ext\sunjce_provider.jar;D:\it\jdk1.8\jre\lib\ext\sunmscapi.jar;D:\it\jdk1.8\jre\lib\ext\sunpkcs11.jar;D:\it\jdk1.8\jre\lib\ext\zipfs.jar;D:\it\jdk1.8\jre\lib\javaws.jar;D:\it\jdk1.8\jre\lib\jce.jar;D:\it\jdk1.8\jre\lib\jfr.jar;D:\it\jdk1.8\jre\lib\jfxswt.jar;D:\it\jdk1.8\jre\lib\jsse.jar;D:\it\jdk1.8\jre\lib\management-agent.jar;D:\it\jdk1.8\jre\lib\plugin.jar;D:\it\jdk1.8\jre\lib\resources.jar;D:\it\jdk1.8\jre\lib\rt.jar;F:\springboot\threaddemo\out\production\threaddemo;D:\it\idea\IntelliJ IDEA 2016.3.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.cky.test.Test3
由B计算count=3
由E计算count=1
由A计算count=5
由D计算count=2
由C计算count=4

Process finished with exit code 0

本实验测试结果分析：尽管pirntln方法在内部是同步的，但i--的操作确是在进入println()之前发生的，所以有发生非线程安全的可能性

 /**
     * Prints a String and then terminate the line.  This method behaves as
     * though it invokes <code>{@link #print(String)}</code> and then
     * <code>{@link #println()}</code>.
     *
     * @param x  The <code>String</code> to be printed.
     */
    public void println(String x) {
        synchronized (this) {
            print(x);
            newLine();
        }
    }