Java 线程优先级

2025 年 3 月 28 日 | 阅读 4 分钟

每个线程都有一个优先级。优先级由1到10之间的数字表示。在大多数情况下，线程调度程序根据线程的优先级调度线程（称为抢占式调度）。但这并不能保证，因为它取决于JVM规范选择哪种调度方式。请注意，Java程序员不仅可以通过JVM在Java程序中显式地为线程分配优先级。

线程优先级的 Setter 和 Getter 方法

让我们讨论线程优先级的 setter 和 getter 方法。

public final int getPriority(): java.lang.Thread.getPriority() 方法返回给定线程的优先级。

public final void setPriority(int newPriority): java.lang.Thread.setPriority() 方法更新或将线程的优先级分配给 newPriority。如果 newPriority 的值超出范围（最小1，最大10），该方法将抛出 IllegalArgumentException。

Thread 类中定义的 3 个常量

public static int MIN_PRIORITY
public static int NORM_PRIORITY
public static int MAX_PRIORITY

线程的默认优先级是 5 (NORM_PRIORITY)。MIN_PRIORITY 的值为 1，MAX_PRIORITY 的值为 10。

线程优先级示例

文件名: ThreadPriorityExample.java

// Importing the required classes
import java.lang.*;

public class ThreadPriorityExample extends Thread 
{

// Method 1
// Whenever the start() method is called by a thread
// the run() method is invoked
public void run()
{
// the print statement
System.out.println("Inside the run() method");
}

// the main method
public static void main(String argvs[])
{
// Creating threads with the help of ThreadPriorityExample class
ThreadPriorityExample th1 = new ThreadPriorityExample();
ThreadPriorityExample th2 = new ThreadPriorityExample();
ThreadPriorityExample th3 = new ThreadPriorityExample();

// We did not mention the priority of the thread.
// Therefore, the priorities of the thread is 5, the default value

// 1st Thread
// Displaying the priority of the thread
// using the getPriority() method
System.out.println("Priority of the thread th1 is : " + th1.getPriority());

// 2nd Thread 
// Display the priority of the thread
System.out.println("Priority of the thread th2 is : " + th2.getPriority());

// 3rd Thread 
// // Display the priority of the thread
System.out.println("Priority of the thread th2 is : " + th2.getPriority());

// Setting priorities of above threads by
// passing integer arguments
th1.setPriority(6);
th2.setPriority(3);
th3.setPriority(9);

// 6
System.out.println("Priority of the thread th1 is : " + th1.getPriority());

// 3
System.out.println("Priority of the thread th2 is : " + th2.getPriority());

// 9
System.out.println("Priority of the thread th3 is : " + th3.getPriority());

// Main thread

// Displaying name of the currently executing thread 
System.out.println("Currently Executing The Thread : " + Thread.currentThread().getName());

System.out.println("Priority of the main thread is : " + Thread.currentThread().getPriority());

// Priority of the main thread is 10 now
Thread.currentThread().setPriority(10);

System.out.println("Priority of the main thread is : " + Thread.currentThread().getPriority());
}
}

输出

Priority of the thread th1 is : 5
Priority of the thread th2 is : 5
Priority of the thread th2 is : 5
Priority of the thread th1 is : 6
Priority of the thread th2 is : 3
Priority of the thread th3 is : 9
Currently Executing The Thread : main
Priority of the main thread is : 5
Priority of the main thread is : 10

我们知道，在线程执行时，高优先级线程将优先于低优先级线程。但是，也存在两个线程具有相同优先级的情况。所有对线程进行管理的处理都由Java线程调度程序完成。请参考以下示例，以了解如果两个线程具有相同的优先级会发生什么。

文件名: ThreadPriorityExample1.java

// importing the java.lang package
import java.lang.*;

public class ThreadPriorityExample1 extends Thread 
{

// Method 1
// Whenever the start() method is called by a thread
// the run() method is invoked
public void run()
{
// the print statement
System.out.println("Inside the run() method");
}


// the main method
public static void main(String argvs[])
{

// Now, priority of the main thread is set to 7
Thread.currentThread().setPriority(7);

// the current thread is retrieved
// using the currentThread() method

// displaying the main thread priority
// using the getPriority() method of the Thread class
System.out.println("Priority of the main thread is : " + Thread.currentThread().getPriority());

// creating a thread by creating an object of the class ThreadPriorityExample1
ThreadPriorityExample1 th1 = new ThreadPriorityExample1();

// th1 thread is the child of the main thread
// therefore, the th1 thread also gets the priority 7

// Displaying the priority of the current thread
System.out.println("Priority of the thread th1 is : " + th1.getPriority());
}
}

输出

Priority of the main thread is : 7
Priority of the thread th1 is : 7

解释: 如果有两个优先级相同的线程，则无法预测哪个线程将首先获得执行机会。执行顺序取决于线程调度程序的算法（先来先服务、轮询等）。

IllegalArgumentException 示例

我们知道，如果 getPriority() 方法的参数 newPriority 的值超出范围（1到10），我们将得到 IllegalArgumentException。让我们通过一个例子来观察。

文件名: IllegalArgumentException.java

// importing the java.lang package
import java.lang.*;

public class IllegalArgumentException extends Thread 
{

// the main method
public static void main(String argvs[])
{

// Now, priority of the main thread is set to 17, which is greater than 10
Thread.currentThread().setPriority(17);

// The current thread is retrieved
// using the currentThread() method

// displaying the main thread priority
// using the getPriority() method of the Thread class
System.out.println("Priority of the main thread is : " + Thread.currentThread().getPriority());

}
}

当我们执行上述程序时，会得到以下异常

Exception in thread "main" java.lang.IllegalArgumentException
	at java.base/java.lang.Thread.setPriority(Thread.java:1141)
	at IllegalArgumentException.main(IllegalArgumentException.java:12)

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Java 线程优先级

线程优先级的 Setter 和 Getter 方法

Thread 类中定义的 3 个常量

线程优先级示例

IllegalArgumentException 示例

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