The core of singleton pattern is to ensure that a class has only one instance and provide a global access point. 1. The basic lazy style may lead to multiple instance creation in a multi-threaded environment, which is not suitable for concurrent scenarios; 2. Although adding a synchronization lock ensures thread safety, locking affects performance every call; 3. Double check locking is optimized through pre-locking judgment and volatile keyword optimization, which is currently recommended; 4. The Hungry style initializes instances during class loading, which is simple and reliable, but does not support delayed loading. In addition, it is necessary to pay attention to the privatization of constructors, prevent reflection attacks and serialization issues. Choose appropriate implementation methods in different scenarios. In most cases, double check locking is enough.
The core of singleton pattern is to ensure that a class has only one instance and provide a global access point. There are many ways to implement it, but the key is thread safety, delayed loading and preventing multiple instantiation.
1. Lazy Initialization
This is the easiest way to implement it, suitable for scenarios where multi-threading is not considered:
public class Singleton {
private static Singleton instance;
private Singleton() {}
public static Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
} Problem : In a multithreaded environment, multiple threads may enter the if (instance == null) branch at the same time, resulting in the creation of multiple instances.
2. Add a synchronization lock to ensure thread safety
If your application is a multi-threaded environment, you can use synchronized to ensure thread safety:
public class Singleton {
private static Singleton instance;
private Singleton() {}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}- This can avoid concurrency problems
- But every time you call
getInstance(), you have to add a lock, and the performance will be affected
3. Double-Checked Locking
To optimize performance, you can make a judgment before locking:
public class Singleton {
private static volatile Singleton instance;
private Singleton() {}
public static Singleton getInstance() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}- The first layer of judgment reduces unnecessary lock competition
- Use
volatilekeywords to ensure variable visibility and prohibit directive reordering - It is currently a more recommended method, taking into account performance and thread safety
4. Eager Initialization
If you don't mind initializing the object in advance, you can use the Hungry style:
public class Singleton {
private static final Singleton INSTANCE = new Singleton();
private Singleton() {}
public static Singleton getInstance() {
return INSTANCE;
}
}advantage:
- Simple and reliable
- The class is initialized when loading, without thread safety issues
shortcoming:
- It is not delayed loading. If the object initialization takes time or takes up resources, it will affect the startup performance.
Other precautions
- Constructor privatization : prevent external instances from being created through new
- Prevent reflection attacks : even if the constructor is private, reflection can destroy singletons. You can add judgments to the constructor to prevent it.
- Serialization and deserialization : If not processed, deserialization may generate a new instance. It is recommended to implement
readResolve()method.
Basically that's it. Just choose different implementation methods in different scenarios. In most cases, double check locking is enough.
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