java怎么写一个单例
时间: 2023-03-19 10:23:45 浏览: 79
要写一个单例,有几种方法:
1. 使用饿汉式单例,在类加载时就创建单例的实例。
```java
public class Singleton {
private static Singleton instance = new Singleton();
private Singleton() {}
public static Singleton getInstance() {
return instance;
}
}
```
2. 使用懒汉式单例,在第一次调用 `getInstance` 方法时创建单例的实例。
```java
public class Singleton {
private static Singleton instance;
private Singleton() {}
public static Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
```
注意:懒汉式单例在多线程环境下可能会出现问题,因此需要加锁来保证线程安全。
```java
public class Singleton {
private static Singleton instance;
private Singleton() {}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
```
3. 使用双重检查加锁 (Double-Checked Locking) 的单例,可以在多线程环境下较好地工作。
```java
public class Singleton {
private volatile static Singleton instance;
private Singleton() {}
public static Singleton getInstance() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}
```
4. 使用静态内部类的单例,可以在多线程环境下较好地工作。
```java
public class Singleton {
private static class SingletonHolder {
private static final Singleton INSTANCE = new Singleton();
}
private Singleton() {}
public static final Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}
```
5. 使用枚举的单例,是最安全