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Java并发编程示例(二)：获取和设置线程信息

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开门见山

Thread类包含几个属性，这些属性所表示的信息能帮助我们识别线程、观察其状态、控制其优先级等。这些线程包括如下几种：

ID： 该属性表示每个线程的唯一标识；
Name： 该属性存储每个线程的名称；
Priority： 该属性存储每个Thread对象的优先级。线程优先级分1到10十个级别，1表示最低优先级，10表示最高优先级。并不推荐修改线程的优先级，但是如果确实有这方面的需求，也可以尝试一下。
Status: 该属性存储线程的状态。线程共有六种不同的状态：新建（new）、运行（runnable）、阻塞（blocked）、等待（waiting）、限时等待（time waiting）或者终止（terminated）。线程的状态必定是其中一种。

在本小节，我们将开发一个程序，程序中新建十个线程，并且设定每个线程的名称和优先级。然后执行线程，观察线程的状态信息，直到线程执行结束。再说明一点，这些线程还是计算一个数的乘法表。

知其然

按照下面所示步骤，来实现该示例：

1.创建一个名为 Calculator的类，实现Runnable接口。代码如下：

public class Calculator implements Runnable {
2.声明一个私有的整形属性，名称为number，实现该类的构造函数来初始化刚刚声明的属性。代码如下：

private int number;
public Calculator(int number) {
this.number = number;
}
3.实现run()方法，该方法是我们创建的线程执行时运行的程序（instruction），故而该方法用于计算乘法表。具体代码如下：

@Override

public void run() {

for (int i = 0; i < 10; i++) {    System.out.printf("%s: %d * %d = %d\n",            Thread.currentThread().getName(),            number, i, i * number);}

}
4.现在，我们来实现示例应用的主类(main class)。创建名为Main的类，在该类中添加main方法。代码如下：

public class Main {

public static void main(String[] args) {}

5.创建两个包含十个元素数组，一个是Thread类型的，一个是Thread.State类型，然后全部初始化。这两个数组，一个用于存储我们将以执行的线程，另外一个存储这些线程的状态。代码如下：

Thread[] threads = new Thread[10];

Thread.State[] status = new Thread.State[threads.length];
6.创建十个Calculator对象，并且使用不同的数来初始化每个对象。使用这些Calculator对象创建十个Thread对象，存储到上面的创建数组中。同时，设置这些线程的优先级，五个设置成最高优先级；五个设置成最低优先级。代码如下：

for (int i = 0; i < threads.length; i++) {

threads[i] = new Thread(new Calculator(i));if ((i % 2) == 0) {    threads[i].setPriority(Thread.MAX_PRIORITY);} else {    threads[i].setPriority(Thread.MIN_PRIORITY);}threads[i].setName("Thread-" + i);

}
7.创建一个PrintWriter对象，用于将线程状态的变换记录到文件中。代码如下：

try (FileWriter file = new FileWriter(“D:\thread.log”);

 PrintWriter pw = new PrintWriter(file)) {}

这里使用了Java7的语法，所以请将JDK升级到第七版，把编译工具设置成Java7。否则会报语法错误。

8.将所有线程的状态写到文件中。现在，现在的状态应该是新建（NEW）。代码如下：

for (int i = 0; i < threads.length; i++) {

Thread thread = threads[i];pw.println("Main: Status of Thread " + i +        " : " + threads[i].getState());status[i] = threads[i].getState();

}

9.启动所有线程。代码入下：

for (int i = 0; i < threads.length; i++) {

threads[i].start();

}
10.另外一方面，我们一直监控线程，直到线程执行结束。如果我们检测到线程的状态有所改变，则立即将线程状态写入到文件中。代码如下：

boolean finish = false;
while (!finish) {
for (int i = 0; i < threads.length; i++) {
if (threads[i].getState() != status[i]) {
writeThreadInfo(pw, threads[i], status[i]);
status[i] = threads[i].getState();
}
}
finish = true;
for (int i = 0; i < threads.length; i++) {
finish = finish
&& (threads[i].getState() == Thread.State.TERMINATED);
}
}
11.实现writeThreadInfo方法，该方法将线程的ID、名称、优先级、旧的状态、新的状态写入到文件中。代码如下：

/**

  • 将一个线程的状态输出到一个文件中。

    *

  • @param pw PrintWriter对象

  • @param thread 需要输出状态的线程对象

  • @param state 线程的旧状态

    */

private static void writeThreadInfo(PrintWriter pw,

                              Thread thread, Thread.State state) {pw.printf("Main : Id %d = %s\n", thread.getId(), thread.getName());pw.printf("Main : Priority: %d\n", thread.getPriority());pw.printf("Main : Old State: %s\n", state);pw.printf("Main : New State: %s\n", thread.getState());pw.printf("Main : ********************************\n");

}
12.运行该示例，然后打开thread.log文件，查看所有线程的演化过程。

知其所以然

下面是thread.log文件的内容片段。从文件内容可以看出，高优先级的线程大致比低优先级的线程较早完成执行。另外，也可以看到每个线程的状态演化过程。

Main: Status of Thread 0 : NEW
Main: Status of Thread 1 : NEW
Main: Status of Thread 2 : NEW
Main: Status of Thread 3 : NEW
Main: Status of Thread 4 : NEW
Main: Status of Thread 5 : NEW
Main: Status of Thread 6 : NEW
Main: Status of Thread 7 : NEW
Main: Status of Thread 8 : NEW
Main: Status of Thread 9 : NEW
Main : Id 10 = Thread-0
Main : Priority: 10
Main : Old State: NEW
Main : New State: RUNNABLE
Main : **********************
Main : Id 11 = Thread-1
Main : Priority: 1
Main : Old State: NEW
Main : New State: BLOCKED
Main : **********************
Main : Id 12 = Thread-2
Main : Priority: 10
Main : Old State: NEW
Main : New State: BLOCKED
Main : **********************
Main : Id 13 = Thread-3
Main : Priority: 1
Main : Old State: NEW
Main : New State: BLOCKED
Main : **********************
Main : Id 14 = Thread-4
Main : Priority: 10
Main : Old State: NEW
Main : New State: BLOCKED
Main : **********************
Main : Id 15 = Thread-5
Main : Priority: 1
Main : Old State: NEW
Main : New State: BLOCKED
Main : **********************
Main : Id 16 = Thread-6
Main : Priority: 10
Main : Old State: NEW
Main : New State: BLOCKED
Main : **********************
Main : Id 17 = Thread-7
Main : Priority: 1
Main : Old State: NEW
Main : New State: BLOCKED
Main : **********************
Main : Id 18 = Thread-8
Main : Priority: 10
Main : Old State: NEW
Main : New State: BLOCKED
Main : **********************
Main : Id 19 = Thread-9
Main : Priority: 1
Main : Old State: NEW
Main : New State: RUNNABLE
Main : **********************
Main : Id 10 = Thread-0
Main : Priority: 10
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
Main : Id 17 = Thread-7
Main : Priority: 1
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 19 = Thread-9
Main : Priority: 1
Main : Old State: RUNNABLE
Main : New State: BLOCKED
Main : **********************
Main : Id 17 = Thread-7
Main : Priority: 1
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
Main : Id 15 = Thread-5
Main : Priority: 1
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 15 = Thread-5
Main : Priority: 1
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
Main : Id 13 = Thread-3
Main : Priority: 1
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 13 = Thread-3
Main : Priority: 1
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
Main : Id 18 = Thread-8
Main : Priority: 10
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 16 = Thread-6
Main : Priority: 10
Main : Old State: BLOCKED
Main : New State: BLOCKED
Main : **********************
Main : Id 14 = Thread-4
Main : Priority: 10
Main : Old State: BLOCKED
Main : New State: BLOCKED
Main : **********************
Main : Id 12 = Thread-2
Main : Priority: 10
Main : Old State: BLOCKED
Main : New State: BLOCKED
Main : **********************
Main : Id 16 = Thread-6
Main : Priority: 10
Main : Old State: BLOCKED
Main : New State: BLOCKED
Main : **********************
Main : Id 18 = Thread-8
Main : Priority: 10
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 18 = Thread-8
Main : Priority: 10
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
Main : Id 11 = Thread-1
Main : Priority: 1
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 11 = Thread-1
Main : Priority: 1
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
Main : Id 19 = Thread-9
Main : Priority: 1
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 16 = Thread-6
Main : Priority: 10
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 19 = Thread-9
Main : Priority: 1
Main : Old State: RUNNABLE
Main : New State: BLOCKED
Main : **********************
Main : Id 16 = Thread-6
Main : Priority: 10
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
Main : Id 19 = Thread-9
Main : Priority: 1
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 14 = Thread-4
Main : Priority: 10
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 19 = Thread-9
Main : Priority: 1
Main : Old State: RUNNABLE
Main : New State: BLOCKED
Main : **********************
Main : Id 14 = Thread-4
Main : Priority: 10
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
Main : Id 19 = Thread-9
Main : Priority: 1
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 12 = Thread-2
Main : Priority: 10
Main : Old State: BLOCKED
Main : New State: RUNNABLE
Main : **********************
Main : Id 12 = Thread-2
Main : Priority: 10
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
Main : Id 19 = Thread-9
Main : Priority: 1
Main : Old State: RUNNABLE
Main : New State: TERMINATED
Main : **********************
下面是控制台的输出片段。输出的是每个线程计算的乘法表，以及所有的线程计算过程。同时，从这里可以更细粒度地看到每个线程的演化过程。

Thread-0: 0 * 0 = 0
Thread-0: 0 * 1 = 0
Thread-0: 0 * 2 = 0
Thread-0: 0 * 3 = 0
Thread-0: 0 * 4 = 0
Thread-0: 0 * 5 = 0
Thread-0: 0 * 6 = 0
Thread-0: 0 * 7 = 0
Thread-0: 0 * 8 = 0
Thread-0: 0 * 9 = 0
Thread-9: 9 * 0 = 0
Thread-9: 9 * 1 = 9
Thread-9: 9 * 2 = 18
Thread-9: 9 * 3 = 27
Thread-9: 9 * 4 = 36
Thread-9: 9 * 5 = 45
Thread-9: 9 * 6 = 54
Thread-7: 7 * 0 = 0
Thread-7: 7 * 1 = 7
Thread-7: 7 * 2 = 14
Thread-7: 7 * 3 = 21
Thread-7: 7 * 4 = 28
Thread-7: 7 * 5 = 35
Thread-7: 7 * 6 = 42
Thread-7: 7 * 7 = 49
Thread-7: 7 * 8 = 56
Thread-7: 7 * 9 = 63
Thread-5: 5 * 0 = 0
Thread-5: 5 * 1 = 5
Thread-5: 5 * 2 = 10
Thread-5: 5 * 3 = 15
Thread-5: 5 * 4 = 20
Thread-5: 5 * 5 = 25
Thread-5: 5 * 6 = 30
Thread-5: 5 * 7 = 35
Thread-5: 5 * 8 = 40
Thread-5: 5 * 9 = 45
Thread-3: 3 * 0 = 0
Thread-3: 3 * 1 = 3
Thread-3: 3 * 2 = 6
Thread-3: 3 * 3 = 9
Thread-3: 3 * 4 = 12
Thread-3: 3 * 5 = 15
Thread-3: 3 * 6 = 18
Thread-3: 3 * 7 = 21
Thread-3: 3 * 8 = 24
Thread-3: 3 * 9 = 27
Thread-1: 1 * 0 = 0
Thread-8: 8 * 0 = 0
Thread-6: 6 * 0 = 0
Thread-4: 4 * 0 = 0
Thread-2: 2 * 0 = 0
Thread-4: 4 * 1 = 4
Thread-6: 6 * 1 = 6
Thread-8: 8 * 1 = 8
Thread-8: 8 * 2 = 16
Thread-8: 8 * 3 = 24
Thread-8: 8 * 4 = 32
Thread-8: 8 * 5 = 40
Thread-8: 8 * 6 = 48
Thread-8: 8 * 7 = 56
Thread-8: 8 * 8 = 64
Thread-8: 8 * 9 = 72
Thread-1: 1 * 1 = 1
Thread-1: 1 * 2 = 2
Thread-1: 1 * 3 = 3
Thread-1: 1 * 4 = 4
Thread-1: 1 * 5 = 5
Thread-1: 1 * 6 = 6
Thread-1: 1 * 7 = 7
Thread-1: 1 * 8 = 8
Thread-1: 1 * 9 = 9
Thread-9: 9 * 7 = 63
Thread-6: 6 * 2 = 12
Thread-4: 4 * 2 = 8
Thread-2: 2 * 1 = 2
Thread-4: 4 * 3 = 12
Thread-6: 6 * 3 = 18
Thread-6: 6 * 4 = 24
Thread-6: 6 * 5 = 30
Thread-6: 6 * 6 = 36
Thread-6: 6 * 7 = 42
Thread-6: 6 * 8 = 48
Thread-6: 6 * 9 = 54
Thread-9: 9 * 8 = 72
Thread-4: 4 * 4 = 16
Thread-2: 2 * 2 = 4
Thread-4: 4 * 5 = 20
Thread-4: 4 * 6 = 24
Thread-4: 4 * 7 = 28
Thread-4: 4 * 8 = 32
Thread-4: 4 * 9 = 36
Thread-9: 9 * 9 = 81
Thread-2: 2 * 3 = 6
Thread-2: 2 * 4 = 8
Thread-2: 2 * 5 = 10
Thread-2: 2 * 6 = 12
Thread-2: 2 * 7 = 14
Thread-2: 2 * 8 = 16
Thread-2: 2 * 9 = 18
Thread类有可以存储线程信息所需的所有属性。Java虚拟机使用线程优先级来每个时刻调度一个线程来使用CPU，并且根据线程的情况来设置其每个线程的状态。

如果没有设置线程的名称，Java虚拟机会使用这种格式来时分配一个名称，Thread-XX，其中XX是一个数字。我们不能修改线程的ID以及线程的状态。Thread类也没有实现setId()和setStatus()方法，以允许做出这些修改。

永无止境

在本节，我们学习了如何使用Thread对象来访问线程信息。其实，Runnable的实现类也运行我们访问这些信息。Thread类的静态方法currentThread()可以获取正在执行的Runnable实现类的对象，进而访问线程的信息。

需要注意的是，如果尝试设置1到10以外的优先级，setPriority()会抛出名为IllegalArgumentException的异常，

拿来主义

本文是从 《Java 7 Concurrency Cookbook》 （D瓜哥窃译为 《Java7并发示例集》 ）翻译而来，仅作为学习资料使用。没有授权，不得用于任何商业行为。

小有所成

Calculator类的完整代码

public class Calculator implements Runnable {
private int number;

public Calculator(int number) {    this.number = number;}@Overridepublic void run() {    for (int i = 0; i < 10; i++) {        System.out.printf("%s: %d * %d = %d\n",                Thread.currentThread().getName(),                number, i, i * number);    }}

}

Test类的完整代码

package com.ysk.demo2;

import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;

public class Test {
public static void main(String[] args) {
Thread[] threads = new Thread[10];
Thread.State[] status = new Thread.State[threads.length];

    for (int i = 0; i < threads.length; i++) {        threads[i] = new Thread(new Calculator(i));        if ((i % 2) == 0) {            threads[i].setPriority(Thread.MAX_PRIORITY);        } else {            threads[i].setPriority(Thread.MIN_PRIORITY);        }        threads[i].setName("Thread-" + i);    }    try (FileWriter file = new FileWriter("D:\\thread.log");         PrintWriter pw = new PrintWriter(file)) {        for (int i = 0; i < threads.length; i++) {            Thread thread = threads[i];            pw.println("Main: Status of Thread " + i +                    " : " + threads[i].getState());            status[i] = threads[i].getState();        }        for (int i = 0; i < threads.length; i++) {            threads[i].start();        }        boolean finish = false;        while (!finish) {            for (int i = 0; i < threads.length; i++) {                if (threads[i].getState() != status[i]) {                    writeThreadInfo(pw, threads[i], status[i]);                    status[i] = threads[i].getState();                }            }            finish = true;            for (int i = 0; i < threads.length; i++) {                finish = finish                        && (threads[i].getState() == Thread.State.TERMINATED);            }        }    } catch (IOException e) {        e.printStackTrace();    }}/** * 将一个线程的状态输出到一个文件中。 * * @param pw     PrintWriter对象 * @param thread 需要输出状态的线程对象 * @param state  线程的旧状态 */private static void writeThreadInfo(PrintWriter pw,Thread thread, Thread.State state) {    pw.printf("Main : Id %d = %s\n",                 thread.getId(), thread.getName());    pw.printf("Main : Priority: %d\n", thread.getPriority());    pw.printf("Main : Old State: %s\n", state);    pw.printf("Main : New State: %s\n", thread.getState());    pw.printf("Main : ********************************\n");}

}

总结

1.线程一共分为以下状态：新建，运行，阻塞，等待，限时等待，终止等6个状态。
2.每个线程有唯一ID，通过getState()方法可以获取线程运行状态,getName()获取线程名称，setPriority()设置线程优先级，默认为5，范围为(1-10)。
3.线程优先级并不能决定哪个线程先执行，只不过，优先级高的线程获取CPU资源的概率较大，优先级低的并非没机会执行。

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