在多线程下懒加载保证只实例化一次。
第一种双重判断：
public class SingletonTest {
private static SingletonTest INSTANCE;
private SingletonTest() {
}
public static SingletonTest getInstance() {
if (INSTANCE == null) {
synchronized (SingletonTest.class) {
if (INSTANCE == null) {
INSTANCE = new SingletonTest();
}
}
}
return INSTANCE;
}
public static void main(String[] args) {
for (int i = 0; i < 100; i++) {
new Thread(() -> {
System.out.println(SingletonTest.getInstance().hashCode());
}).start();
}
}
}
第二种内部类，JVM会保证内部类只加载一次：
public class SingletonTest {
private SingletonTest() {
}
private static class SingletonTestInnerClass {
private static final SingletonTest INSTANCE = new SingletonTest();
}
public static SingletonTest getInstance() {
return SingletonTestInnerClass.INSTANCE;
}
public static void main(String[] args) {
for (int i = 0; i < 100; i++) {
new Thread(() -> {
System.out.println(SingletonTest.getInstance().hashCode());
}).start();
}
}
}
第三种使用枚举：
public enum SingletonTestEnum {
INSTANCE;
public void test() {
}
public static void main(String[] args) {
for (int i = 0; i < 100; i++) {
new Thread(() -> {
System.out.println(SingletonTestEnum.INSTANCE.hashCode());
}).start();
}
}
}
注：这些是自己学习的笔记，如有不准确，欢迎指出！
原文：https://www.cnblogs.com/seaming/p/14905079.html