【源起Netty 外传】ServiceLoader详解

小灰灰 2022-04-24 06:40 139阅读 0赞

# [【源起Netty 外传】ServiceLoader详解][Netty _ServiceLoader] #

*  [![868271510-54cb382abb7a1_small][] java][868271510-54cb382abb7a1_small_java]

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### 前戏 ###

netty源码注释有云：

...
    
    If a provider class has been installed in a jar file that is
    visible to the system class loader, and that jar file contains a
    provider-configuration file named
    <tt>java.nio.channels.spi.SelectorProvider</tt> in the resource
    directory <tt>META-INF/services</tt>, then the first class name
    specified in that file is taken.  The class is loaded and
    instantiated; if this process fails then an unspecified error is
    thrown.
    
    ...

不知所云？好吧，重点是，jar文件中在`META-INF/services`目录下配置了A属性，以某种方式能加载到。

*how can do that?* 答曰：**ServiceLoader**

### 摸索 ###

`ServiceLoader`直译为`服务加载器`，最终目的是获取service的impl实现类。正如前文所说，它将加载`META-INF/services`下的配置文件，来锁定impl实现类。

*  代码走起

**1.首先，要有一个接口**

//形状接口
    public interface Shape {
        String introduce(); //介绍
    }

**2.然后，要有该接口的实现类。**实现类很简单，介绍自己的形状是啥

//实现类一
    public class Circle implements Shape {
        public String introduce() {
            return "圆形";  //言简意赅的介绍
        }
    }

//实现类二
    public class Sequare implements Shape {
        static{
            System.out.println("【Sequare】据说有延时加载，try it..");
        }
        public String introduce() {
            return "方形";
        }
    }

眼尖的朋友可能已经注意到了，这里有个**静态块**，因为资料中有提到ServiceLoader有延时加载的效果。寡人不信，遂验之……

**3.配置文件，放在**`META-INF/services`

文件位置：

- src
        -main
            -resources
                - META-INF
                    - services
                        - xxxpackage.Shape

文件名：**包名.接口名**

文件内容：**包名.接口实现类，换行符分隔**

xxxpackage.Circle
    xxxpackage.Sequare

**4.ServiceLoader调用**

ServiceLoader<Shape> shapeLoader = ServiceLoader.load(Shape.class);
    Iterator<Shape> it = shapeLoader.iterator();
    while(it.hasNext()){
        System.out.println("Iterator<Shape> next()方法调用..");
        Shape shape = it.next();
        System.out.printf("what's shape?%s\n",shape.introduce());
    }

调用结果：

Iterator<Shape> next()方法调用..
    【Sequare】据说有延时加载，try it..
    what's shape?方形

从该结果可看出，在调用`it.next()`的时候，才真正的加载了`Sequare`类，确确实实是**延时加载**。期具体实现依靠ServiceLoader的内部类LazyIterator，感性趣的朋友可以自己看一下。

### 总结 ###

1.  ServiceLoader重点在于**可跨越jar包**获取impl，这一点笔者通过maven多模块项目亲测ok
2.  **延时加载**特性

[Netty _ServiceLoader]: https://segmentfault.com/a/1190000012895516
[868271510-54cb382abb7a1_small]: /images/20220218/45002c845c914452b1c4ca045d82e55a.png
[868271510-54cb382abb7a1_small_java]: https://segmentfault.com/t/java/blogs