单例模式

对于资源

package singleton;

public record Resource(String name) {
}

饿汉式

package singleton;

public final class HungrySingleton {
    private static final Resource INSTANCE = new Resource("hungry");

    public static Resource getInstance() {
        return INSTANCE;
    }

    private HungrySingleton() {
    }
}

懒汉式

package singleton;

public final class LazySingleton {
    private static volatile Resource INSTANCE;

    public static synchronized Resource getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Resource("lazy");
        }
        return INSTANCE;
    }

    public static Resource getInstanceV2() {
        if (INSTANCE == null) {
            synchronized (LazySingleton.class) {
                if (INSTANCE == null) {
                    INSTANCE = new Resource("lazy v2");
                }
            }
        }
        return INSTANCE;
    }

    private LazySingleton() {
    }
}

静态内部类（JVM 特性）

package singleton;

public final class InnerClassSingleton {
    public static Resource getInstance() {
        return InstanceHolder.INSTANCE;
    }

    private static class InstanceHolder {
        private static final Resource INSTANCE = new Resource("inner class");
    }

    private InnerClassSingleton() {
    }
}

枚举类（JVM 特性）

package singleton;

public enum EnumSingleton {
    INSTANCE;

    private final Resource resource;

    EnumSingleton() {
        this.resource = new Resource("enum");
    }

    public Resource getResource() {
        return resource;
    }
}