读Dubbo源码，学习SPI

核心类

ExtensionLoader

使用方法

1. 定义接口，使用@SPI标记

@SPI("impl1")
public interface SimpleExt {
    // @Adaptive example, do not specify a explicit key.
    @Adaptive
    String echo(URL url, String s);

    @Adaptive({"key1", "key2"})
    String yell(URL url, String s);

    // no @Adaptive
    String bang(URL url, int i);
}

@SPI("impl1")
public interface UseProtocolKeyExt {
    // protocol key is the second
    @Adaptive({"key1", "protocol"})
    String echo(URL url, String s);

    // protocol key is the first
    @Adaptive({"protocol", "key2"})
    String yell(URL url, String s);
}

2. 扩展类

• SimpleExt.java

public class SimpleExtImpl1 implements SimpleExt {
    public String echo(URL url, String s) {
        return "Ext1Impl1-echo";
    }

    public String yell(URL url, String s) {
        return "Ext1Impl1-yell";
    }

    public String bang(URL url, int i) {
        return "bang1";
    }
}

public class SimpleExtImpl2 implements SimpleExt {
    public String echo(URL url, String s) {
        return "Ext1Impl2-echo";
    }

    public String yell(URL url, String s) {
        return "Ext1Impl2-yell";
    }

    public String bang(URL url, int i) {
        return "bang2";
    }

}

public class SimpleExtImpl3 implements SimpleExt {
    public String echo(URL url, String s) {
        return "Ext1Impl3-echo";
    }

    public String yell(URL url, String s) {
        return "Ext1Impl3-yell";
    }

    public String bang(URL url, int i) {
        return "bang3";
    }

}

---

• UseProtocolKeyExt.java

public class UseProtocolKeyExtImpl1 implements UseProtocolKeyExt {
    public String echo(URL url, String s) {
        return "Ext3Impl1-echo";
    }

    public String yell(URL url, String s) {
        return "Ext3Impl1-yell";
    }
}

public class UseProtocolKeyExtImpl2 implements UseProtocolKeyExt {
    public String echo(URL url, String s) {
        return "Ext3Impl2-echo";
    }

    public String yell(URL url, String s) {
        return "Ext3Impl2-yell";
    }
}

public class UseProtocolKeyExtImpl3 implements UseProtocolKeyExt {
    public String echo(URL url, String s) {
        return "Ext3Impl3-echo";
    }

    public String yell(URL url, String s) {
        return "Ext3Impl3-yell";
    }
}

3.SPI资源路径

• 路径-> META-INFO/dubbo/interal/{@SPI注解的全限定名}

com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt

• 实现类配置

  • com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt

  impl1=com.alibaba.dubbo.common.extensionloader.ext1.impl.SimpleExtImpl1#Hello World impl2=com.alibaba.dubbo.common.extensionloader.ext1.impl.SimpleExtImpl2 # Comment 2 impl3=com.alibaba.dubbo.common.extensionloader.ext1.impl.SimpleExtImpl3 # with head space

  • com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt

  impl1=com.alibaba.dubbo.common.extensionloader.ext3.impl.UseProtocolKeyExtImpl1 impl2=com.alibaba.dubbo.common.extensionloader.ext3.impl.UseProtocolKeyExtImpl2 impl3=com.alibaba.dubbo.common.extensionloader.ext3.impl.UseProtocolKeyExtImpl3

4.测试方法

• SimpleExt.java

    @Test
    public void test_getAdaptiveExtension_defaultAdaptiveKey() throws Exception {
        {
            // #1
            SimpleExt ext = ExtensionLoader.getExtensionLoader(SimpleExt.class).getAdaptiveExtension();

            Map<String, String> map = new HashMap<String, String>();
            URL url = new URL("p1", "1.2.3.4", 1010, "path1", map);
            // #2
            String echo = ext.echo(url, "haha");
            assertEquals("Ext1Impl1-echo", echo);
        }

        {
            #3
            SimpleExt ext = ExtensionLoader.getExtensionLoader(SimpleExt.class).getAdaptiveExtension();

            Map<String, String> map = new HashMap<String, String>();
            map.put("simple.ext", "impl2");
            URL url = new URL("p1", "1.2.3.4", 1010, "path1", map);
            
            #4
            String echo = ext.echo(url, "haha");
            assertEquals("Ext1Impl2-echo", echo);
        }
    }

• #1.该方法执行后会通过ExtensionLoader.createAdaptiveExtensionClassCode生成一个代理类对象ext，见附录1
• #2.1 ext中有一行：String extName = url.getParameter("simple.ext", "impl1"); (key,defaultValue) ，其中key：simple.ext是接口名称SimpleExt去驼峰加. 构成，原因在于echo方法没有@Adaptive注解没有传入参数；defaultValue根据接口@SPI值impl1生成。
• #2.2 url中map为空，extName取传入的默认值impl1
• #2.3 ext中有一行 com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt extension = (com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt.class).getExtension(extName); 根据extName扩展名去寻找真正需要的扩展实现类。此时extName是impl1，那么真正执行的echo就是impl1代表的SimpleExtImpl1实例
• #3同#1
• #4.1 url中map为("simple.ext", "impl2")
• #4.2 通过String extName = url.getParameter("simple.ext", "impl1")；获得extName为impl2
• #4.3 此时extName是impl2，那么真正执行的echo就是impl2代表的SimpleExtImpl2实例

    @Test
    public void test_getAdaptiveExtension_customizeAdaptiveKey() throws Exception {
        
        SimpleExt ext = ExtensionLoader.getExtensionLoader(SimpleExt.class).getAdaptiveExtension();

        Map<String, String> map = new HashMap<String, String>();
        map.put("key2", "impl2");
        URL url = new URL("p1", "1.2.3.4", 1010, "path1", map);
        #5
        String echo = ext.yell(url, "haha");
        assertEquals("Ext1Impl2-yell", echo);
        #6
        url = url.addParameter("key1", "impl3"); // note: URL is value's type
        echo = ext.yell(url, "haha");
        assertEquals("Ext1Impl3-yell", echo);
    }

• #5.1 因为yell方法上@Adaptive注解有参数{"key1", "key2"}，那么ext生成的代理方法中获取extName代码为：String extName = url.getParameter("key1",url.getParameter("key2", "impl1"));
• #5.2 url中map为("key2", "impl2")，第一轮判后extName是impl2，第二轮判断后extName为impl2
• #5.3 此时extName是impl2，那么真正执行的echo就是impl2代表的SimpleExtImpl2实例
• #6.1 url中map为("key2", "impl2")("key1", "impl3") ,第一轮判后extName是impl2，第二轮判断后extName为impl3
• #6.2 此时extName是impl3，那么真正执行的echo就是impl3代表的SimpleExtImpl3实例
• 参数判断顺序与参数定义顺序相反

    @Test
    public void test_getAdaptiveExtension_protocolKey() throws Exception {
        #1
        UseProtocolKeyExt ext = ExtensionLoader.getExtensionLoader(UseProtocolKeyExt.class).getAdaptiveExtension();
    
        {
            #2
            String echo = ext.echo(URL.valueOf("1.2.3.4:20880"), "s");
            assertEquals("Ext3Impl1-echo", echo); // default value
            
            #3
            Map<String, String> map = new HashMap<String, String>();
            URL url = new URL("impl3", "1.2.3.4", 1010, "path1", map);
    
            echo = ext.echo(url, "s");
            assertEquals("Ext3Impl3-echo", echo); // use 2nd key, protocol
    
            #4
            url = url.addParameter("key1", "impl2");
            echo = ext.echo(url, "s");
            assertEquals("Ext3Impl2-echo", echo); // use 1st key, key1
        }

• #1.该方法执行后会通过ExtensionLoader.createAdaptiveExtensionClassCode生成一个代理类对象ext，见附录2
• #2.1 echo方法@Adaptive注解中有值({"key1", "protocol"})，且其中一个为protocol，在ext中extName判断方法为url.getParameter("key1", (url.getProtocol() == null ? "impl1" : url.getProtocol()));
• #2.2 url中未指定protocol，同时map为null，判断后extName为默认值impl1
• #2.3 此时extName是impl1，那么真正执行的echo就是impl1代表的UseProtocolKeyExtImpl1实例
• #3.1 此时url的protocol为impl3，extName为impl3
• #3.2 此时extName是impl3，那么真正执行的echo就是impl3代表的UseProtocolKeyExtImpl3实例
• #4.1 此时url的proto是impl3，map("key1", "impl2")，extName是impl2
• #4.2 此时extName是impl2，那么真正执行的echo就是impl2代表的UseProtocolKeyExtImpl2实例

        {
    
            Map<String, String> map = new HashMap<String, String>();
            URL url = new URL(null, "1.2.3.4", 1010, "path1", map);
            #5
            String yell = ext.yell(url, "s");
            assertEquals("Ext3Impl1-yell", yell); // default value
            
            #6
            url = url.addParameter("key2", "impl2"); // use 2nd key, key2
            yell = ext.yell(url, "s");
            assertEquals("Ext3Impl2-yell", yell);
            #7
            url = url.setProtocol("impl3"); // use 1st key, protocol
            yell = ext.yell(url, "d");
            assertEquals("Ext3Impl3-yell", yell);
        }
    }

• #5.1 yell方法@Adaptive注解中有值({"protocol", "key1"})，且其中一个为protocol，在ext中extName判断方法为url.getProtocol() == null ? (url.getParameter( "key2", "impl1" ) ) : url.getProtocol()
• #5.2 url中未指定protocol，同时map为null，判断后extName为默认值impl1
• #5.3 此时extName是impl1，那么真正执行的echo就是impl1代表的UseProtocolKeyExtImpl1实例
• #6.1 此时map("key2", "impl2"),extName为impl2
• #6.2 此时extName是impl2，那么真正执行的echo就是impl2代表的UseProtocolKeyExtImpl2实例
• #7.1 此时url的proto是impl3，map("key1", "impl2")，extName是impl3
• #7.2 此时extName是impl3，那么真正执行的echo就是impl3代表的UseProtocolKeyExtImpl3实例

附录1

package com.alibaba.dubbo.common.extensionloader.ext1;

import com.alibaba.dubbo.common.extension.ExtensionLoader;


public class SimpleExt$Adaptive implements com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt {
    public java.lang.String echo(com.alibaba.dubbo.common.URL arg0,
        java.lang.String arg1) {
        if (arg0 == null) {
            throw new IllegalArgumentException("url == null");
        }

        com.alibaba.dubbo.common.URL url = arg0;
        String extName = url.getParameter("simple.ext", "impl1");

        if (extName == null) {
            throw new IllegalStateException(
                "Fail to get extension(com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt) name from url(" +
                url.toString() + ") use keys([simple.ext])");
        }

        com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt extension = (com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt.class)
                                                                                                                                                     .getExtension(extName);

        return (extension.echo(arg0, arg1));
    }

    public java.lang.String yell(com.alibaba.dubbo.common.URL arg0,
        java.lang.String arg1) {
        if (arg0 == null) {
            throw new IllegalArgumentException("url == null");
        }

        com.alibaba.dubbo.common.URL url = arg0;
        String extName = url.getParameter("key1",
                url.getParameter("key2", "impl1"));

        if (extName == null) {
            throw new IllegalStateException(
                "Fail to get extension(com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt) name from url(" +
                url.toString() + ") use keys([key1, key2])");
        }

        com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt extension = (com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt.class)
                                                                                                                                                     .getExtension(extName);

        return (extension.yell(arg0, arg1));
    }

    public java.lang.String bang(com.alibaba.dubbo.common.URL arg0, int arg1) {
        throw new UnsupportedOperationException(
            "method public abstract java.lang.String com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt.bang(com.alibaba.dubbo.common.URL,int) of interface com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt is not adaptive method!");
    }
}

附录2

package com.alibaba.dubbo.common.extensionloader.ext3;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class UseProtocolKeyExt$Adaptive implements com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt {
	public java.lang.String echo( com.alibaba.dubbo.common.URL arg0, java.lang.String arg1 )
	{
		if ( arg0 == null )
			throw new IllegalArgumentException( "url == null" );
		com.alibaba.dubbo.common.URL	url	= arg0;
		String				extName = url.getParameter( "key1", (url.getProtocol() == null ? "impl1" : url.getProtocol() ) );
		if ( extName == null )
			throw new IllegalStateException( "Fail to get extension(com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt) name from url(" + url.toString() + ") use keys([key1, protocol])" );
		com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt extension = 
		(com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt)ExtensionLoader.getExtensionLoader( com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt.class ).getExtension( extName );
		return(extension.echo( arg0, arg1 ) );
	}


	public java.lang.String yell( com.alibaba.dubbo.common.URL arg0, java.lang.String arg1 )
	{
		if ( arg0 == null )
			throw new IllegalArgumentException( "url == null" );
		com.alibaba.dubbo.common.URL	url	= arg0;
		String				extName = url.getProtocol() == null ? (url.getParameter( "key2", "impl1" ) ) : url.getProtocol();
		if ( extName == null )
			throw new IllegalStateException( "Fail to get extension(com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt) name from url(" + url.toString() + ") use keys([protocol, key2])" );
		com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt extension = (com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt)ExtensionLoader.
		getExtensionLoader( com.alibaba.dubbo.common.extensionloader.ext3.UseProtocolKeyExt.class ).getExtension( extName );
		return(extension.yell( arg0, arg1 ) );
	}
}