Java 多线程
Java线程教程 - Java多线程以下代码显示了如何在程序中运行多线程。public class Main {
public static void main(String[] args) {
// Create two Thread objects
Thread t1 = new Thread(Main::print);
Thread t2 = new Thread(Main::print);
// Start both threadst1.start();
t2.start();
}
public static void print() {
for (int i = 1; i <= 500; i++) {
System.out.println(i);
}
}
}
上面的代码生成以下结果。线程同步Java编程语言内置了两种线程同步：互斥同步条件同步在互斥同步中，在一个时间点只允许一个线程访问代码段。条件同步通过条件变量和三个操作来实现：等待，信号和广播。同步关键字synchronized关键字用于声明需要同步的关键部分。有两种方法可以使用synchronized关键字：将方法声明为关键部分将语句块声明为关键段我们可以通过在方法的返回类型之前使用关键字synchronized来声明一个方法作为临界段。public class Main {
public synchronized void someMethod_1() {
// Method code goes here
}
public static synchronized void someMethod_2() {
// Method code goes here
}
}
我们可以声明一个实例方法和一个静态方法同步。构造函数不能声明为同步。以下代码说明了使用关键字synchronized：public class Main {
public synchronized void someMethod_1() {
// only one thread can execute here at a time
}
public void someMethod_11() {
synchronized (this) {
// only one thread can execute here at a time
}
}
public void someMethod_12() {
// multiple threads can execute here at a time
synchronized (this) {
// only one thread can execute here at a time
}
// multiple threads can execute here at a time
}
public static synchronized void someMethod_2() {
// only one thread can execute here at a time
}
public static void someMethod_21() {
synchronized (Main.class) {
// only one thread can execute here at a time
}
}
public static void someMethod_22() {
// multiple threads can execute here at a time
synchronized (Main.class) {
// only one thread can execute here at a time
}
// multiple threads can execute here at a time
}
}
wait()方法对wait()方法的调用必须放在synchronized方法或同步块中。对于当前线程已经获取监视器的对象，必须调用wait()方法。notify()方法没有办法唤醒等待集中的特定线程。例子public class Main {
private static int myValue = 1;
public static void main(String[] args) {
Thread t = new Thread(() -> {
while (true) {
updateBalance();
}
});
t.start();
t = new Thread(() -> {
while (true) {
monitorBalance();
}
});
t.start();
}
public static synchronized void updateBalance() {
System.out.println("start:" + myValue);
myValue = myValue + 1;
myValue = myValue - 1;
System.out.println("end:" + myValue);
}
public static synchronized void monitorBalance() {
int b = myValue;
if (b != 1) {
System.out.println("Balance changed: " + b);
System.exit(1);
}
}
}
上面的代码生成以下结果。例2以下代码显示了上述代码的非同步版本。public class Main {
private static int myValue = 1;
public static void main(String[] args) {
Thread t = new Thread(() -> {
while (true) {
updateBalance();
}
});
t.start();
t = new Thread(() -> {
while (true) {
monitorBalance();
}
});
t.start();
}
public static void updateBalance() {
System.out.println("start:" + myValue);
myValue = myValue + 1;
myValue = myValue - 1;
System.out.println("end:" + myValue);
}
public static synchronized void monitorBalance() {
int b = myValue;
if (b != 1) {
System.out.println("Balance changed: " + b);
System.exit(1);
}
}
}
上面的代码生成以下结果。