单例模式*

public class Singleton {
    
    private static volatile Singleton instance = null;//防止指令重排序，拿到半成品对象
 
    private Singleton() {}
 
    public static Singleton getInstance() {
        if(null == instance) {//避免每次都进行锁竞争，提高执行效率
            synchronized(Singleton.class) {//加锁
                if(null == instance) {//防止多个线程越过第一个if后多次创建对象
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

public class Singleton {
 
    private Singleton() {}
 
    private static class SingletonHolder {
        public final static Singleton instance = new Singleton();
    }
 
    public static Singleton getInstance() {
        return SingletonHolder.instance;
    }
}

import com.google.common.base.Suppliers;
import com.google.gson.Gson;

import java.util.function.Supplier;

public final class GsonInstance {

    private static Supplier<Gson> gsonSupplier = Suppliers.memoize(Gson::new)::get;

    public static Gson getGsonInstance() {
        return gsonSupplier.get();
    }
}