103-创建并启动线程

/**
 * 读数据
 */
private static void readFromDataBase() {
    // read data from database and handle it
    try {
        println("Begin read data from db.");
        Thread.sleep(1000 * 5L);
        println("Read data done and start handle it.");
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    println("The data handle finish and successfully.");
}

/**
 * 写数据
 */
private static void writeDataToFile() {
    // write data to file
    try {
        println("Begin write data to file.");
        Thread.sleep(1000 * 10L);
        println("Write data done and start handle it.");
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    println("The data handle finish and successfully.");
}

1. 顺序执行，而非交替执行

   // 例1
   readFromDataBase();
   writeDataToFile();
   // 例2
   for (int i = 0; i < 100; i++) {
       println("Task i=>" + i);
   }
   for (int j = 0; j < 100; j++) {
       println("Task j=>" + j);
   }

2. 交替执行

   Thread t1 = new Thread("Custom-Thread") {
       @Override
       public void run() {
           for (int i = 0; i < 100; i++) {
               println("Task i=>" + i);
               try {
                   Thread.sleep(1000 * 1L);
               } catch (InterruptedException e) {
                   e.printStackTrace();
               }
           }
       }
   };
   t1.start();
   for (int j = 0; j < 100; j++) {
       println("Task j=>" + j);
   }

   

3. 交替执行读写操作

   new Thread("READ-Thread") {
       @Override
       public void run() {
           readFromDataBase();
       }
   }.start();
   new Thread("WRITE-Thread") {
       @Override
       public void run() {
           writeDataToFile();
       }
   }.start();

• 创建线程后不调用start()方法，此时他是一个线程么？

  不是。涉及到线程的生命周期。Thread被new出来之后只是一个Java实例，和其他普通Java实例一模一样。只有.start()启动之后才会变为一个线程，而且启动是立即返回的，不是BLOCKED阻塞。

JDK文档：https://docs.oracle.com/javase/8/docs/api/

查看java.lang.Thread

A thread is a thread of execution in a program. The Java Virtual Machine allows an application to have multiple threads of execution running concurrently.

Every thread has a priority. Threads with higher priority are executed in preference to threads with lower priority. Each thread may or may not also be marked as a daemon.

When a Java Virtual Machine starts up, there is usually a single non-daemon thread (which typically calls the method named main of some designated class).

There are two ways to create a new thread of execution. One is to declare a class to be a subclass of Thread. This subclass should override the run method of class Thread. An instance of the subclass can then be allocated and started.

验证启动程序后JVM会自动创建main线程：

1. 编写程序并运行

   public static void main(String[] args) {
       try {
           Thread.sleep(1000 * 100L);
       } catch (InterruptedException e) {
           e.printStackTrace();
       }
   }

2. 显示当前所有Java进程pid

   jps

3. 基于JMX的可视化监视、管理工具。实时的监控Java程序在运行过程中的内存、CPU、线程的使用情况，并可以对加载的相关类进行分析

   jconsole <thread_pid>

   

4. 总结：

   JVM启动时：

   • 非守护线程：main()整个程序的入口。
   • 守护线程：Finalizer()垃圾回收器、Reference Handler引用具柄、Signal Dispatcher接收系统信号命令、JMX、RMI