1.4isAlive()方法

方法isAlive()的功能是判断当前线程是否处于活动状态

活动状态是线程已经启动且尚未终止，线程处于正在运行或准备开始运行的状态，就认为线程是存活的。

测试如下

package com.cky.thread;

/**
 * Created by edison on 2017/11/28.
 */
public class MyThread9 extends Thread {
    @Override
    public void run() {
        super.run();
        System.out.println("run="+this.isAlive());
    }
}

package com.cky.test;

import com.cky.thread.MyThread9;

/**
 * Created by edison on 2017/11/28.
 */
public class Test14 {
    public static void main(String[] args) throws InterruptedException {
        MyThread9 th = new MyThread9();
        System.out.println("begin="+th.isAlive());
        th.start();
       // Thread.sleep(1000);
        System.out.println("end="+th.isAlive());

    }
}

C:itsoftjdkinjava -Didea.launcher.port=7534 "-Didea.launcher.bin.path=C:itsoftideaIntelliJ IDEA 2016.3.3in" -Dfile.encoding=UTF-8 -classpath "C:itsoftjdkjrelibcharsets.jar;C:itsoftjdkjrelibdeploy.jar;C:itsoftjdkjrelibextaccess-bridge-32.jar;C:itsoftjdkjrelibextcldrdata.jar;C:itsoftjdkjrelibextdnsns.jar;C:itsoftjdkjrelibextjaccess.jar;C:itsoftjdkjrelibextjfxrt.jar;C:itsoftjdkjrelibextlocaledata.jar;C:itsoftjdkjrelibext
ashorn.jar;C:itsoftjdkjrelibextsunec.jar;C:itsoftjdkjrelibextsunjce_provider.jar;C:itsoftjdkjrelibextsunmscapi.jar;C:itsoftjdkjrelibextsunpkcs11.jar;C:itsoftjdkjrelibextzipfs.jar;C:itsoftjdkjrelibjavaws.jar;C:itsoftjdkjrelibjce.jar;C:itsoftjdkjrelibjfr.jar;C:itsoftjdkjrelibjfxswt.jar;C:itsoftjdkjrelibjsse.jar;C:itsoftjdkjrelibmanagement-agent.jar;C:itsoftjdkjrelibplugin.jar;C:itsoftjdkjrelib
esources.jar;C:itsoftjdkjrelib
t.jar;C:多线程核心技术第一章outproduction第一章;C:itsoftideaIntelliJ IDEA 2016.3.3libidea_rt.jar" com.intellij.rt.execution.application.AppMain com.cky.test.Test14
begin=false
end=true
run=true

Process finished with exit code 0

结果分析：上面刚开始还没有执行start()方法，所以begin为false,当开启线程之后，先打印end，说明此时cpu切换到了主线程，这时执行end，开启的线程还没有执行完毕，所以end为true,最后开启的线程

执行完run函数的代码就结束了。

下面更改代码如下

package com.cky.test;

import com.cky.thread.MyThread9;

/**
 * Created by edison on 2017/11/28.
 */
public class Test14 {
    public static void main(String[] args) throws InterruptedException {
        MyThread9 th = new MyThread9();
        System.out.println("begin="+th.isAlive());
        th.start();
        Thread.sleep(1000);
        System.out.println("end="+th.isAlive());

    }
}

C:itsoftjdkinjava -Didea.launcher.port=7535 "-Didea.launcher.bin.path=C:itsoftideaIntelliJ IDEA 2016.3.3in" -Dfile.encoding=UTF-8 -classpath "C:itsoftjdkjrelibcharsets.jar;C:itsoftjdkjrelibdeploy.jar;C:itsoftjdkjrelibextaccess-bridge-32.jar;C:itsoftjdkjrelibextcldrdata.jar;C:itsoftjdkjrelibextdnsns.jar;C:itsoftjdkjrelibextjaccess.jar;C:itsoftjdkjrelibextjfxrt.jar;C:itsoftjdkjrelibextlocaledata.jar;C:itsoftjdkjrelibext
ashorn.jar;C:itsoftjdkjrelibextsunec.jar;C:itsoftjdkjrelibextsunjce_provider.jar;C:itsoftjdkjrelibextsunmscapi.jar;C:itsoftjdkjrelibextsunpkcs11.jar;C:itsoftjdkjrelibextzipfs.jar;C:itsoftjdkjrelibjavaws.jar;C:itsoftjdkjrelibjce.jar;C:itsoftjdkjrelibjfr.jar;C:itsoftjdkjrelibjfxswt.jar;C:itsoftjdkjrelibjsse.jar;C:itsoftjdkjrelibmanagement-agent.jar;C:itsoftjdkjrelibplugin.jar;C:itsoftjdkjrelib
esources.jar;C:itsoftjdkjrelib
t.jar;C:多线程核心技术第一章outproduction第一章;C:itsoftideaIntelliJ IDEA 2016.3.3libidea_rt.jar" com.intellij.rt.execution.application.AppMain com.cky.test.Test14
begin=false
run=true
end=false

Process finished with exit code 0

结果分析：上面执行start方法之后，调用了sleep方法，此时主线程被迫休眠，cpu切换到了子线程，这时执行完了run函数的代码，子线程已经关闭，这时cpu再切换回主线程，执行最后的end方法，但

此刻子线程已经关闭，不是存活状态。说明子线程在1秒内就执行完了。

在使用isAlive方法时，如果将线程对象以构造参数的方式传递给Thread对象进行start()启动，运行的结果是有差异的，造成这个差异的主要原因还是Thread.currentThread()和this的差异

测试代码如下

package com.cky.thread;

/**
 * Created by edison on 2017/11/28.
 */
public class CountOperate2 extends  Thread{
    public CountOperate2 () {
        System.out.println("count 。。。 begin");
        System.out.println("thread name="+ Thread.currentThread().getName());
        System.out.println("thread current isalive="+ Thread.currentThread().isAlive());
        System.out.println("this.name="+ this.getName());
        System.out.println("this.isalive="+ this.isAlive());
        System.out.println("count ... end");
    }

    @Override
    public void run() {
        super.run();
        System.out.println("run 。。。 begin");
        System.out.println("thread name="+ Thread.currentThread().getName());
        System.out.println("thread current isalive="+ Thread.currentThread().isAlive());
        System.out.println("this.name="+ this.getName());
        System.out.println("this.isalive="+ this.isAlive());
        System.out.println("run ... end");
    }
}

package com.cky.test;

import com.cky.thread.CountOperate2;

/**
 * Created by edison on 2017/11/28.
 */
public class Test15 {
    public static void main(String[] args) {
        CountOperate2 cc = new CountOperate2();
        Thread t1 = new Thread(cc);
        System.out.println("main begin t1 isalive="+t1.isAlive());
        t1.start();
        System.out.println("main end t1 isalive="+t1.isAlive());
    }
}

C:itsoftjdkinjava -Didea.launcher.port=7536 "-Didea.launcher.bin.path=C:itsoftideaIntelliJ IDEA 2016.3.3in" -Dfile.encoding=UTF-8 -classpath "C:itsoftjdkjrelibcharsets.jar;C:itsoftjdkjrelibdeploy.jar;C:itsoftjdkjrelibextaccess-bridge-32.jar;C:itsoftjdkjrelibextcldrdata.jar;C:itsoftjdkjrelibextdnsns.jar;C:itsoftjdkjrelibextjaccess.jar;C:itsoftjdkjrelibextjfxrt.jar;C:itsoftjdkjrelibextlocaledata.jar;C:itsoftjdkjrelibext
ashorn.jar;C:itsoftjdkjrelibextsunec.jar;C:itsoftjdkjrelibextsunjce_provider.jar;C:itsoftjdkjrelibextsunmscapi.jar;C:itsoftjdkjrelibextsunpkcs11.jar;C:itsoftjdkjrelibextzipfs.jar;C:itsoftjdkjrelibjavaws.jar;C:itsoftjdkjrelibjce.jar;C:itsoftjdkjrelibjfr.jar;C:itsoftjdkjrelibjfxswt.jar;C:itsoftjdkjrelibjsse.jar;C:itsoftjdkjrelibmanagement-agent.jar;C:itsoftjdkjrelibplugin.jar;C:itsoftjdkjrelib
esources.jar;C:itsoftjdkjrelib
t.jar;C:多线程核心技术第一章outproduction第一章;C:itsoftideaIntelliJ IDEA 2016.3.3libidea_rt.jar" com.intellij.rt.execution.application.AppMain com.cky.test.Test15
count 。。。 begin
thread name=main
thread current isalive=true
this.name=Thread-0
this.isalive=false
count ... end
main begin t1 isalive=false
main end t1 isalive=true
run 。。。 begin
thread name=Thread-1
thread current isalive=true
this.name=Thread-0
this.isalive=false
run ... end

Process finished with exit code 0

结果分析：

首先执行主函数的时候，实例化线程对象会执行构造函数，所以打印的Thread.currentThread().isAlive是true,但是这个这个线程对象还在实例化，并没有执行start（）方法，所以this.isAlive为false,后面打印

t1也是false,之后执行了start()方法，子线程开启，cpu还是切换到了main线程，先执行main函数代码，打印t1为true,之后cpu切换到了子线程，执行run函数的代码，这时Thread.currentThread为true,但是this

却打印为false。个人理解为t1还是A还是存活的，但是由于他是由一个线程对象传递进去的，而里面的这个线程对象也就是thread-0已经不再是存活状态。