EnityManagerFactory 单例。请检查一下？

发布于 2024-11-07 17:43:09 字数 768 浏览 0 评论 0原文

我只是好奇这看起来是否可靠。它没有给出任何错误，但我只是想仔细检查一下，因为我遇到了 c3p0 的池问题。只是检查一下是否有任何原因。先感谢您！

import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;


public class EntityManagerFactorySingleton {
    private static EntityManagerFactorySingleton singleton;
    private EntityManagerFactory emf;

public EntityManagerFactorySingleton(){
    emf = Persistence.createEntityManagerFactory(ConfigList.getProperty(Config.PERSISTENCE_UNIT), System.getProperties());
}

public synchronized static EntityManagerFactorySingleton getInstance() {
   if(singleton == null) {
    singleton = new EntityManagerFactorySingleton();
   }
   return singleton;
}

public EntityManagerFactory getEntityManagerFactory(){
    return emf;
}

}

原文

I am just cruious if this looks solid. It gives no errors but I just want to double check as I am having a pooling issue with c3p0. Just checking to see if anything here is the cause. Thank you in advance!

import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;


public class EntityManagerFactorySingleton {
    private static EntityManagerFactorySingleton singleton;
    private EntityManagerFactory emf;

public EntityManagerFactorySingleton(){
    emf = Persistence.createEntityManagerFactory(ConfigList.getProperty(Config.PERSISTENCE_UNIT), System.getProperties());
}

public synchronized static EntityManagerFactorySingleton getInstance() {
   if(singleton == null) {
    singleton = new EntityManagerFactorySingleton();
   }
   return singleton;
}

public EntityManagerFactory getEntityManagerFactory(){
    return emf;
}

}

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江湖彼岸 2024-11-14 17:43:09

您的代码不是“可靠的”：

构造函数对于单例来说必须是私有的，
您不应该同步 getInstance() 方法，尽管您需要执行线程安全的初始化。这是因为初始化后，所有需要该实例的线程都必须相互等待（这是一个无用的瓶颈）。
仅当您的实例为 null 时，才调用执行初始化的同步（私有）方法；在该方法中，再次检查实例是否为 null。另一种方法是使用一个私有内部类 SingletonHolder 来保存实例，因此您将依赖类加载器来执行线程安全初始化。

但是，如果您无法（不想）避免使用单例，那么一个非常好的选择是仅定义一个常量的枚举：INSTANCE；

public enum EntityManagerFactorySingleton {
    INSTANCE;

    // all your code -- fields, constructor, instance / static methods get in here
    // you can still have the `getInstance()` static method returning INSTANCE, if you want.
}

唯一的缺点是您无法对 INSTANCE 执行延迟初始化，但您现在是线程安全的，并且可以轻松应对序列化或克隆问题。

Your code is not "solid":

constructor must be private for a singleton
you shouldn't have the getInstance() method synchronised, although you need to perform the initialisation thread-safe. That's because after initialization, all the threads that need the instance will have to wait for each other (and that's a useless bottleneck).
Only if your instance is null, call a synchronized (private) method that performs the initialisation; inside that method, check again if the instance is null. Another approach is to have a private inner class SingletonHolder that holds the instance, so you'll rely on the class-loader for performing the thread-safe initialisation.

However, if you can't (don't want to) avoid using a singleton, a very good choice would be an enum with only one constant defined: INSTANCE;

public enum EntityManagerFactorySingleton {
    INSTANCE;

    // all your code -- fields, constructor, instance / static methods get in here
    // you can still have the `getInstance()` static method returning INSTANCE, if you want.
}

The only drawback is that you cannot perform lazy initialisation for the INSTANCE, but you're now thread-safe and ready for serialization or cloning issues without any effort.

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