Java 简明教程
Java - Multithreading
Java 是一种多线程编程语言，这意味着我们可以使用 Java 开发多线程程序。多线程程序包含两个或更多部分，它们可以同时运行，并且每个部分可以同时处理不同的任务，充分利用可用资源，特别是在计算机具有多个 CPUs 时。
Java is a multi-threaded programming language which means we can develop multi-threaded program using Java. A multi-threaded program contains two or more parts that can run concurrently and each part can handle a different task at the same time making optimal use of the available resources specially when your computer has multiple CPUs.
根据定义，多任务处理是多个进程共享公共处理资源（如 CPU）时发生的情况。多线程将多任务处理的概念扩展到应用程序中，您可以在其中将单个应用程序内的特定操作细分为各个线程。每个线程都可以并行运行。操作系统不仅在不同应用程序之间划分处理时间，还在应用程序内的每个线程之间划分处理时间。
By definition, multitasking is when multiple processes share common processing resources such as a CPU. Multi-threading extends the idea of multitasking into applications where you can subdivide specific operations within a single application into individual threads. Each of the threads can run in parallel. The OS divides processing time not only among different applications, but also among each thread within an application.
Java Multithreading
多线程使您能够以一种方式编写，以便在同一程序中可以同时进行多个活动。要实现多线程（或编写多线程代码），您需要 java.lang.Thread class 。
Multi-threading enables you to write in a way where multiple activities can proceed concurrently in the same program. To achieve the multithreading (or, write multithreaded code), you need java.lang.Thread class.
Life Cycle of a Thread in Java Multithreading
线程在其生命周期中经过不同的阶段。例如，一个线程诞生、启动、运行，然后死亡。下图显示了线程的完整生命周期。
A thread goes through various stages in its life cycle. For example, a thread is born, started, runs, and then dies. The following diagram shows the complete life cycle of a thread.
以下是生命周期的阶段 -
Following are the stages of the life cycle −
-
New − A new thread begins its life cycle in the new state. It remains in this state until the program starts the thread. It is also referred to as a born thread.
-
Runnable − After a newly born thread is started, the thread becomes runnable. A thread in this state is considered to be executing its task.
-
Waiting − Sometimes, a thread transitions to the waiting state while the thread waits for another thread to perform a task. A thread transitions back to the runnable state only when another thread signals the waiting thread to continue executing.
-
Timed Waiting − A runnable thread can enter the timed waiting state for a specified interval of time. A thread in this state transitions back to the runnable state when that time interval expires or when the event it is waiting for occurs.
-
Terminated (Dead) − A runnable thread enters the terminated state when it completes its task or otherwise terminates.
Thread Priorities
每个 Java 线程都有一个优先级，这有助于操作系统确定线程调度的顺序。
Every Java thread has a priority that helps the operating system determine the order in which threads are scheduled.
Java 线程优先级介于 MIN_PRIORITY（常量为 1）和 MAX_PRIORITY（常量为 10）之间。默认情况下，每个线程都获得 NORM_PRIORITY（常量为 5）优先级。
Java thread priorities are in the range between MIN_PRIORITY (a constant of 1) and MAX_PRIORITY (a constant of 10). By default, every thread is given priority NORM_PRIORITY (a constant of 5).
具有较高优先级的线程对程序而言更为重要，并且应该在较低优先级的线程之前分配处理器时间。但是，线程优先级不能保证线程执行的顺序，并且很大程度上取决于平台。
Threads with higher priority are more important to a program and should be allocated processor time before lower-priority threads. However, thread priorities cannot guarantee the order in which threads execute and are very much platform dependent.
Create a Thread by Implementing a Runnable Interface
如果准备将您的类作为线程执行，那么可以通过实现 Runnable 接口实现此目的。您需要遵循三个基本步骤 −
If your class is intended to be executed as a thread then you can achieve this by implementing a Runnable interface. You will need to follow three basic steps −
Step 1: Implement run() Method
第一步，您需要实现 Runnable 接口提供的 run() 方法。此方法为线程提供了一个入口点，您需要将完整业务逻辑放入此方法内。以下是 run() 方法的一个简单语法：
As a first step, you need to implement a run() method provided by a Runnable interface. This method provides an entry point for the thread and you will put your complete business logic inside this method. Following is a simple syntax of the run() method −
public void run( )Step 2: Instantiate a Thread Object
作为第二步，您将使用以下构造函数实例化一个 Thread 对象 −
As a second step, you will instantiate a Thread object using the following constructor −
Thread(Runnable threadObj, String threadName);其中，threadObj 是实现 Runnable 接口的类的实例，threadName 是赋予新线程的名称。
Where, threadObj is an instance of a class that implements the Runnable interface and threadName is the name given to the new thread.
Step 3: Call Thread using start() Method
一旦创建了 Thread 对象，就可以通过调用 start() 方法来启动它，它执行对 run( ) 方法的调用。以下是 start() 方法的简单语法 -
Once a Thread object is created, you can start it by calling start() method, which executes a call to run( ) method. Following is a simple syntax of start() method −
void start();Example: Create Thread by Implementing Runnable Interface
下面是创建一个新线程并开始运行它的示例：
Here is an example that creates a new thread and starts running it −
class RunnableDemo implements Runnable {
private Thread t;
private String threadName;
RunnableDemo( String name) {
threadName = name;
System.out.println("Creating " + threadName );
}
public void run() {
System.out.println("Running " + threadName );
try {
for(int i = 4; i > 0; i--) {
System.out.println("Thread: " + threadName + ", " + i);
// Let the thread sleep for a while.
Thread.sleep(50);
}
} catch (InterruptedException e) {
System.out.println("Thread " + threadName + " interrupted.");
}
System.out.println("Thread " + threadName + " exiting.");
}
public void start () {
System.out.println("Starting " + threadName );
if (t == null) {
t = new Thread (this, threadName);
t.start ();
}
}
}
public class TestThread {
public static void main(String args[]) {
RunnableDemo R1 = new RunnableDemo( "Thread-1");
R1.start();
RunnableDemo R2 = new RunnableDemo( "Thread-2");
R2.start();
}
}Output
Creating Thread-1
Starting Thread-1
Creating Thread-2
Starting Thread-2
Running Thread-1
Thread: Thread-1, 4
Running Thread-2
Thread: Thread-2, 4
Thread: Thread-1, 3
Thread: Thread-2, 3
Thread: Thread-1, 2
Thread: Thread-2, 2
Thread: Thread-1, 1
Thread: Thread-2, 1
Thread Thread-1 exiting.
Thread Thread-2 exiting.Create a Thread by Extending a Thread Class
创建线程的第二种方法是创建一个扩展 Thread 类的类，使用以下两个简单步骤。这种方法在使用 Thread 类中可用方法创建多个线程时提供了更多的灵活性。
The second way to create a thread is to create a new class that extends Thread class using the following two simple steps. This approach provides more flexibility in handling multiple threads created using available methods in Thread class.
Step 1: Override run() Method
您需要覆盖 Thread 类中可用的 run( ) 方法。此方法为线程提供一个入口点，您将把您的完整业务逻辑放在这个方法中。以下是 run() 方法的简单语法：
You will need to override run( ) method available in Thread class. This method provides an entry point for the thread and you will put your complete business logic inside this method. Following is a simple syntax of run() method −
public void run( )Step 2: Call Thread using start() Method
一旦创建 Thread 对象，便可以通过调用 start() 方法来启动它，该调用会执行一个 run( ) 方法调用。以下是 start() 方法的一个简单语法 −
Once Thread object is created, you can start it by calling start() method, which executes a call to run( ) method. Following is a simple syntax of start() method −
void start( );Example: Create Thread by Extending Thread Class
以下是按照 Thread 扩展重写的先前程序：
Here is the preceding program rewritten to extend the Thread −
class ThreadDemo extends Thread {
private Thread t;
private String threadName;
ThreadDemo( String name) {
threadName = name;
System.out.println("Creating " + threadName );
}
public void run() {
System.out.println("Running " + threadName );
try {
for(int i = 4; i > 0; i--) {
System.out.println("Thread: " + threadName + ", " + i);
// Let the thread sleep for a while.
Thread.sleep(50);
}
} catch (InterruptedException e) {
System.out.println("Thread " + threadName + " interrupted.");
}
System.out.println("Thread " + threadName + " exiting.");
}
public void start () {
System.out.println("Starting " + threadName );
if (t == null) {
t = new Thread (this, threadName);
t.start ();
}
}
}
public class TestThread {
public static void main(String args[]) {
ThreadDemo T1 = new ThreadDemo( "Thread-1");
T1.start();
ThreadDemo T2 = new ThreadDemo( "Thread-2");
T2.start();
}
}Output
Creating Thread-1
Starting Thread-1
Creating Thread-2
Starting Thread-2
Running Thread-1
Thread: Thread-1, 4
Running Thread-2
Thread: Thread-2, 4
Thread: Thread-1, 3
Thread: Thread-2, 3
Thread: Thread-1, 2
Thread: Thread-2, 2
Thread: Thread-1, 1
Thread: Thread-2, 1
Thread Thread-1 exiting.
Thread Thread-2 exiting.Thread Methods
以下是 Thread 类中提供的重要方法列表：
Following is the list of important methods available in the Thread class.
Sr.No. |
Method & Description |
1 |
public void start() Starts the thread in a separate path of execution, then invokes the run() method on this Thread object. |
2 |
public void run() If this Thread object was instantiated using a separate Runnable target, the run() method is invoked on that Runnable object. |
3 |
public final void setName(String name) Changes the name of the Thread object. There is also a getName() method for retrieving the name. |
4 |
public final void setPriority(int priority) Sets the priority of this Thread object. The possible values are between 1 and 10. |
5 |
public final void setDaemon(boolean on) A parameter of true denotes this Thread as a daemon thread. |
6 |
public final void join(long millisec) The current thread invokes this method on a second thread, causing the current thread to block until the second thread terminates or the specified number of milliseconds passes. |
7 |
public void interrupt() Interrupts this thread, causing it to continue execution if it was blocked for any reason. |
8 |
public final boolean isAlive() Returns true if the thread is alive, which is any time after the thread has been started but before it runs to completion. |
之前的方法都是在特定 Thread 对象上调用的。Thread 类中的以下方法是静态的。调用其中一个静态方法对当前运行的线程执行该操作。
The previous methods are invoked on a particular Thread object. The following methods in the Thread class are static. Invoking one of the static methods performs the operation on the currently running thread.
Sr.No. |
Method & Description |
1 |
public static void yield() Causes the currently running thread to yield to any other threads of the same priority that are waiting to be scheduled. |
2 |
public static void sleep(long millisec) Causes the currently running thread to block for at least the specified number of milliseconds. |
3 |
public static boolean holdsLock(Object x) Returns true if the current thread holds the lock on the given Object. |
4 |
public static Thread currentThread() Returns a reference to the currently running thread, which is the thread that invokes this method. |
5 |
public static void dumpStack() Prints the stack trace for the currently running thread, which is useful when debugging a multithreaded application. |
Example
以下 ThreadClassDemo 程序演示了 Thread 类的其中一些方法。考虑一下 DisplayMessage 类，它实现了 Runnable −
The following ThreadClassDemo program demonstrates some of these methods of the Thread class. Consider a class DisplayMessage which implements Runnable −
// File Name : DisplayMessage.java
// Create a thread to implement Runnable
public class DisplayMessage implements Runnable {
private String message;
public DisplayMessage(String message) {
this.message = message;
}
public void run() {
while(true) {
System.out.println(message);
}
}
}以下是另一个扩展 Thread 类的类−
Following is another class which extends the Thread class −
// File Name : GuessANumber.java
// Create a thread to extentd Thread
public class GuessANumber extends Thread {
private int number;
public GuessANumber(int number) {
this.number = number;
}
public void run() {
int counter = 0;
int guess = 0;
do {
guess = (int) (Math.random() * 100 + 1);
System.out.println(this.getName() + " guesses " + guess);
counter++;
} while(guess != number);
System.out.println("** Correct!" + this.getName() + "in" + counter + "guesses.**");
}
}以下是使用上述定义类的主程序−
Following is the main program, which makes use of the above-defined classes −
// File Name : ThreadClassDemo.java
public class ThreadClassDemo {
public static void main(String [] args) {
Runnable hello = new DisplayMessage("Hello");
Thread thread1 = new Thread(hello);
thread1.setDaemon(true);
thread1.setName("hello");
System.out.println("Starting hello thread...");
thread1.start();
Runnable bye = new DisplayMessage("Goodbye");
Thread thread2 = new Thread(bye);
thread2.setPriority(Thread.MIN_PRIORITY);
thread2.setDaemon(true);
System.out.println("Starting goodbye thread...");
thread2.start();
System.out.println("Starting thread3...");
Thread thread3 = new GuessANumber(27);
thread3.start();
try {
thread3.join();
} catch (InterruptedException e) {
System.out.println("Thread interrupted.");
}
System.out.println("Starting thread4...");
Thread thread4 = new GuessANumber(75);
thread4.start();
System.out.println("main() is ending...");
}
}
class DisplayMessage implements Runnable {
private String message;
public DisplayMessage(String message) {
this.message = message;
}
public void run() {
while(true) {
System.out.println(message);
}
}
}
class GuessANumber extends Thread {
private int number;
public GuessANumber(int number) {
this.number = number;
}
public void run() {
int counter = 0;
int guess = 0;
do {
guess = (int) (Math.random() * 100 + 1);
System.out.println(this.getName() + " guesses " + guess);
counter++;
} while(guess != number);
System.out.println("** Correct!" + this.getName() + "in" + counter + "guesses.**");
}
}