『Dubbo SPI源码分析』@Adaptive 机制分析

『Dubbo SPI源码分析』@Adaptive 机制分析

• 基于 2.7.0 版本
• 上一章：『Dubbo SPI源码分析』依赖注入机制分析

1. 创建测试 demo

• 首先创建一个接口，举例为 Car

package com.luban.dubbo_spi.api;

@SPI
public interface Car {
    public void getColor();
    
    @Adaptive
    public void getColorForUrl(URL url);
}
- 根据接口，先创建一个实现类
```java
package com.luban.dubbo_spi.impl;

public class RedCar implements Car {

    public void getColor() {
        System.out.println("red");
    }
    
    @Override
    public void getColorForUrl(URL url) {
    }
}

• 再创建一个实现类

package com.luban.dubbo_spi.impl;

public class BlackCar implements Car {

    public void getColor() {
        System.out.println("black");
    }

    @Override
    public void getColorForUrl(URL url) {
        System.out.println("blackUrl");
    }
}

• 然后在 resources 创建一个目录 META-INF.services 目录
• 并在 META-INF.services 目录中创建文件 com.luban.dubbo_spi.api.Car 一定要与接口同名

red = com.luban.dubbo_spi.impl.RedCar
black = com.luban.dubbo_spi.impl.BlackCar

• 创建主程序测试 @SPI

public class CarDemo {

    public static void main(String[] args) {

        ExtensionLoader<Car> carLoader = ExtensionLoader.getExtensionLoader(Car.class);
        Car adCar = carLoader.getAdaptiveExtension();

        Map<String, String> map = new HashMap<>();
        map.put("car", "black");
        URL url = new URL("","",1, map);
        adCar.getColorForUrl(url);
    }
}

2. @Adaptive 除了可以标记在自定义的 Adaptive 类（即 com.luban.dubbo_spi.impl.AdaptiveCar 类）上面以外（『Dubbo SPI源码分析』依赖注入机制分析），还可以标记在接口的对应方法中，获取 dubbo 帮你实现的代理类，首先也是按照老套路，先执行 ExtensionLoader.getExtensionLoader() 扫描配置文件中实现了 com.luban.dubbo_spi.api.Car 的类以后，就可以从中获取到 Adaptive 扩展类

public class CarDemo {

    public static void main(String[] args) {
		// 1. 获取实现了 com.luban.dubbo_spi.api.Car 接口的扩展类
        ExtensionLoader<Car> carLoader = ExtensionLoader.getExtensionLoader(Car.class);
        // 2. 获取 Adaptive 扩展类
        Car adCar = carLoader.getAdaptiveExtension();
        ...
    }
}

3. com.luban.dubbo_spi.api.Car 的 ExtensionLoader 先判断是否有过 Adaptive 实例对象，如果有则直接返回，没有就创建

public class ExtensionLoader<T> {
    private final Holder<Object> cachedAdaptiveInstance = new Holder<Object>();
    private volatile Throwable createAdaptiveInstanceError;
    ...
    public T getAdaptiveExtension() {
        // 一个接口只能有一个Adaptive实现
        Object instance = cachedAdaptiveInstance.get();
        if (instance == null) {
            if (createAdaptiveInstanceError == null) {
                synchronized (cachedAdaptiveInstance) {
                    // 1. 尝试从缓存中获取 Adaptive 对象
                    instance = cachedAdaptiveInstance.get();
                    if (instance == null) {
                        try {
                            // 2. 没有就创建
                            instance = createAdaptiveExtension(); 
                            cachedAdaptiveInstance.set(instance);
                        } catch (Throwable t) {
                            createAdaptiveInstanceError = t;
                            throw new IllegalStateException("fail to create adaptive instance: " + t.toString(), t);
                        }
                    }
                }
            } else {
                throw new IllegalStateException("fail to create adaptive instance: " + createAdaptiveInstanceError.toString(), createAdaptiveInstanceError);
            }
        }

        return (T) instance;
    }
}

4. ExtensionLoader 会调用 getAdaptiveExtensionClass() 获取 Adaptive 扩展类，其中依赖注入就忽略了，因为配置的 case 没有依赖注入

public class ExtensionLoader<T> {
    ...
    @SuppressWarnings("unchecked")
    private T createAdaptiveExtension() {
        try {
            // 1. 获取 Adaptive 扩展类
            return injectExtension((T) getAdaptiveExtensionClass().newInstance());
        } catch (Exception e) {
            throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e);
        }
    }
}

5. ExtensionLoader 继续调用 createAdaptiveExtensionClass() 方法，用于生成代理类

public class ExtensionLoader<T> {
    ...
    private Class<?> createAdaptiveExtensionClass() {
    	// 1. 生成代理类的代码
        String code = createAdaptiveExtensionClassCode();
		...
    }
}

6. 首先遍历每个方法，判断存在一个方法有注释 @Adaptive，没有则抛异常。找到有注释的方法后，先找到存在 URL.class 入参的位置，生成校验 URL 入参代码。然后从 @Adaptive 的 value 属性中取值，没有的话，只能用类名小写代替，即为 “car”（这个参数是 URL 上的占位符 key，用作取数据）。然后判断方法参数列表是否有 org.apache.dubbo.rpc.Invocation 入参，有也生成相应的校验入参代码。最后根据 value 值，生成获取参数的代码，并返回

• 重点
  • @Adaptive 注解中，如果填写了 value 值，代表指定占位符名称，没填写，则用类名小写
  • 一定要填入 URL.class 参数，除非自定义其他 protocol 协议
  • 没标注 @Adaptive 注解的方法，使用会抛异常
  • 可以再添加一个 org.apache.dubbo.rpc.Invocation 入参
• 生成的代理代码如下

package com.luban.dubbo_spi.api;
import org.apache.dubbo.common.extension.ExtensionLoader;
public class Car$Adaptive implements com.luban.dubbo_spi.api.Car {
    public void getColorForUrl(org.apache.dubbo.common.URL arg0) {
        if (arg0 == null) throw new IllegalArgumentException("url == null");
        org.apache.dubbo.common.URL url = arg0;
        String extName = url.getParameter("car");
        if(extName == null) throw new IllegalStateException("Fail to get extension(com.luban.dubbo_spi.api.Car) name from url(" + url.toString() + ") use keys([car])");
        com.luban.dubbo_spi.api.Car extension = (com.luban.dubbo_spi.api.Car)ExtensionLoader.getExtensionLoader(com.luban.dubbo_spi.api.Car.class).getExtension(extName);extension.getColorForUrl(arg0);
    }
    public void getColor() {throw new UnsupportedOperationException("method public abstract void com.luban.dubbo_spi.api.Car.getColor() of interface com.luban.dubbo_spi.api.Car is not adaptive method!");
    }
}

• 源代码

public class ExtensionLoader<T> {
	private final Class<?> type;
	private String cachedDefaultName;
	...
    private String createAdaptiveExtensionClassCode() {
        StringBuilder codeBuilder = new StringBuilder();
        Method[] methods = type.getMethods();
        boolean hasAdaptiveAnnotation = false;
        // 1. 看是不是所有方法上都没有 Adaptive 注解，都没有则会抛异常
        for (Method m : methods) {
            if (m.isAnnotationPresent(Adaptive.class)) {
                hasAdaptiveAnnotation = true;
                break;
            }
        }
        // 没有 Adaptive 注解，就抛异常
        if (!hasAdaptiveAnnotation) {
            throw new IllegalStateException("No adaptive method on extension " + type.getName() + ", refuse to create the adaptive class!");
        }

        // 生成包，import, 类名
        codeBuilder.append("package ").append(type.getPackage().getName()).append(";");
        codeBuilder.append("\nimport ").append(ExtensionLoader.class.getName()).append(";");
        codeBuilder.append("\npublic class ").append(type.getSimpleName()).append("$Adaptive").append(" implements ").append(type.getCanonicalName()).append(" {");

        for (Method method : methods) {
            Class<?> rt = method.getReturnType();
            Class<?>[] pts = method.getParameterTypes();
            Class<?>[] ets = method.getExceptionTypes();

            Adaptive adaptiveAnnotation = method.getAnnotation(Adaptive.class);
            StringBuilder code = new StringBuilder(512);
            // 2. 如果方法上没有 Adaptive 注解，则加上抛异常代码
            if (adaptiveAnnotation == null) {
                code.append("throw new UnsupportedOperationException(\"method ")
                        .append(method.toString()).append(" of interface ")
                        .append(type.getName()).append(" is not adaptive method!\");");
            } else {
                // 3. 寻找 URL 参数所在位置
                int urlTypeIndex = -1;
                for (int i = 0; i < pts.length; ++i) {
                    if (pts[i].equals(URL.class)) {
                        urlTypeIndex = i;
                        break;
                    }
                }
                
                // 4. 存在 URL 参数，就从 URL 中取数据，并判空
                if (urlTypeIndex != -1) {
                    // Null Point check
                    String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"url == null\");",
                            urlTypeIndex);
                    code.append(s);

                    s = String.format("\n%s url = arg%d;", URL.class.getName(), urlTypeIndex);
                    code.append(s);
                }
                else {
                    String attribMethod = null;
                    // 如果没有 URL 参数，就从其他参数中去找是不是有返回值为 URL 的 get 方法
                    LBL_PTS:
                    for (int i = 0; i < pts.length; ++i) {
                        Method[] ms = pts[i].getMethods();
                        for (Method m : ms) {
                            String name = m.getName();
                            if ((name.startsWith("get") || name.length() > 3)
                                    && Modifier.isPublic(m.getModifiers())
                                    && !Modifier.isStatic(m.getModifiers())
                                    && m.getParameterTypes().length == 0
                                    && m.getReturnType() == URL.class) {
                                urlTypeIndex = i;
                                attribMethod = name;
                                break LBL_PTS;
                            }
                        }
                    }
                    if (attribMethod == null) {
                        throw new IllegalStateException("fail to create adaptive class for interface " + type.getName()
                                + ": not found url parameter or url attribute in parameters of method " + method.getName());
                    }

                    // Null point check
                    String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"%s argument == null\");",
                            urlTypeIndex, pts[urlTypeIndex].getName());
                    code.append(s);
                    s = String.format("\nif (arg%d.%s() == null) throw new IllegalArgumentException(\"%s argument %s() == null\");",
                            urlTypeIndex, attribMethod, pts[urlTypeIndex].getName(), attribMethod);
                    code.append(s);

                    s = String.format("%s url = arg%d.%s();", URL.class.getName(), urlTypeIndex, attribMethod);
                    code.append(s);
                }
                // 5. 取 adaptive 注解的值，可以是个数组，如果没有值，则会取类名，LoadBalance->load.balance
                String[] value = adaptiveAnnotation.value();
                if (value.length == 0) {
                    String splitName = StringUtils.camelToSplitName(type.getSimpleName(), ".");
                    value = new String[]{splitName};
                }
                // 6. 判断参数列表中是否有 Invocation 对象
                boolean hasInvocation = false;
                for (int i = 0; i < pts.length; ++i) {
                    if (("org.apache.dubbo.rpc.Invocation").equals(pts[i].getName())) {
                        // Null Point check
                        String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"invocation == null\");", i);
                        code.append(s);
                        s = String.format("\nString methodName = arg%d.getMethodName();", i);
                        code.append(s);
                        hasInvocation = true;
                        break;
                    }
                }
                // 默认名是接口上的 Extension 注解上对于的值
                String defaultExtName = cachedDefaultName;
                String getNameCode = null;
                // 倒序循环 adaptive 注解的值
                for (int i = value.length - 1; i >= 0; --i) {
                    if (i == value.length - 1) {
                        if (null != defaultExtName) {
                            if (!"protocol".equals(value[i])) {
                                if (hasInvocation) {
                                    // url.getMethodParameter(methodName, value[i], defaultExtName)
                                    // methodName 是从 Invocation 对象中获取的
                                    getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                                } else {
                                    // url.getParameter(value[i], defaultExtName)
                                    getNameCode = String.format("url.getParameter(\"%s\", \"%s\")", value[i], defaultExtName);
                                }
                            } else {
                                getNameCode = String.format("( url.getProtocol() == null ? \"%s\" : url.getProtocol() )", defaultExtName);
                            }
                        } else {
                            if (!"protocol".equals(value[i])) {
                                if (hasInvocation) {
                                    getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                                } else {
                                     
                                    getNameCode = String.format("url.getParameter(\"%s\")", value[i]);
                                }
                            } else {
                                getNameCode = "url.getProtocol()";
                            }
                        }
                    } else {
                        if (!"protocol".equals(value[i])) {
                            if (hasInvocation) {
                                getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                            } else {
                                // 7. 生成取从 URL 取数据代码，其中 value[i] = car
                                getNameCode = String.format("url.getParameter(\"%s\", %s)", value[i], getNameCode);
                            }
                        } else {
                            getNameCode = String.format("url.getProtocol() == null ? (%s) : url.getProtocol()", getNameCode);
                        }
                    }
                }
                code.append("\nString extName = ").append(getNameCode).append(";");
                // check extName == null?
                String s = String.format("\nif(extName == null) " +
                                "throw new IllegalStateException(\"Fail to get extension(%s) name from url(\" + url.toString() + \") use keys(%s)\");",
                        type.getName(), Arrays.toString(value));
                code.append(s);

                // 根据取出来的扩展名获取扩展
                s = String.format("\n%s extension = (%,
                        type.getName(), ExtensionLoader.class.getSimpleName(), type.getName());
                code.append(s);

                // return statement
                if (!rt.equals(void.class)) {
                    code.append("\nreturn ");
                }
                // 执行扩展实现类的方法
                s = String.format("extension.%s(", method.getName());
                code.append(s);
                for (int i = 0; i < pts.length; i++) {
                    if (i != 0) {
                        code.append(", ");
                    }
                    code.append("arg").append(i);
                }
                code.append(");");
            }

            codeBuilder.append("\npublic ").append(rt.getCanonicalName()).append(" ").append(method.getName()).append("(");
            for (int i = 0; i < pts.length; i++) {
                if (i > 0) {
                    codeBuilder.append(", ");
                }
                codeBuilder.append(pts[i].getCanonicalName());
                codeBuilder.append(" ");
                codeBuilder.append("arg").append(i);
            }
            codeBuilder.append(")");
            if (ets.length > 0) {
                codeBuilder.append(" throws ");
                for (int i = 0; i < ets.length; i++) {
                    if (i > 0) {
                        codeBuilder.append(", ");
                    }
                    codeBuilder.append(ets[i].getCanonicalName());
                }
            }
            codeBuilder.append(" {");
            codeBuilder.append(code.toString());
            codeBuilder.append("\n}");
        }
        codeBuilder.append("\n}");
        if (logger.isDebugEnabled()) {
            logger.debug(codeBuilder.toString());
        }
        // 8. 返回生成的代码
        return codeBuilder.toString();
    }
}

7. 当生成代理类代码后，通过获取加载器，以及编译器来生成对应代码，并返回

public class ExtensionLoader<T> {
    ...
    private Class<?> createAdaptiveExtensionClass() {
    	// 生成代理类的代码
        String code = createAdaptiveExtensionClassCode();
        // 1. 创建加载器
        ClassLoader classLoader = findClassLoader();
        // 2. 创建编译器
        org.apache.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(org.apache.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
        // 3. 编译生成代码
        return compiler.compile(code, classLoader);
    }
}

8. 代码生成之后，要传入 URL 参数，以获取实际执行的类，其中参数一定是 “car”，而 value 值是在 com.luban.dubbo_spi.api.Car 配置文件的类

public class CarDemo {

    public static void main(String[] args) {

        ExtensionLoader<Car> carLoader = ExtensionLoader.getExtensionLoader(Car.class);
        Car adCar = carLoader.getAdaptiveExtension();

        Map<String, String> map = new HashMap<>();
        map.put("car", "black");
        URL url = new URL("","",1, map);
        // 1. 传入 URL 参数，执行方法
        adCar.getColorForUrl(url);
    }
}

9. 实际执行的时候，就是执行下面这个代理类了，其中会根据 URL 的参数，通过 getExtensionLoader() 获取指定的类并且实例化后执行 getColorForUrl() 方法

package com.luban.dubbo_spi.api;
import org.apache.dubbo.common.extension.ExtensionLoader;
public class Car$Adaptive implements com.luban.dubbo_spi.api.Car {
    public void getColorForUrl(org.apache.dubbo.common.URL arg0) {
        if (arg0 == null) throw new IllegalArgumentException("url == null");
        org.apache.dubbo.common.URL url = arg0;
        String extName = url.getParameter("car");
        if(extName == null) throw new IllegalStateException("Fail to get extension(com.luban.dubbo_spi.api.Car) name from url(" + url.toString() + ") use keys([car])");
        com.luban.dubbo_spi.api.Car extension = (com.luban.dubbo_spi.api.Car)ExtensionLoader.getExtensionLoader(com.luban.dubbo_spi.api.Car.class).getExtension(extName);extension.getColorForUrl(arg0);
    }
    public void getColor() {throw new UnsupportedOperationException("method public abstract void com.luban.dubbo_spi.api.Car.getColor() of interface com.luban.dubbo_spi.api.Car is not adaptive method!");
    }
}

10. 总结