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这篇文章主要讲解了“Dubbo的SPI机制介绍以及Adaptive实例”，文中的讲解内容简单清晰，易于学习与理解，下面请大家跟着小编的思路慢慢深入，一起来研究和学习“Dubbo的SPI机制介绍以及Adaptive实例”吧！
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@SPI("dubbo")
public interface AdaptiveExt {
@Adaptive
// 单元测试方法4的注解为@Adaptive({"t"})
String echo(String msg, URL url);
}public class DubboAdaptiveExt implements AdaptiveExt {
@Override
public String echo(String msg, URL url) {
return "dubbo";
}
}
public class SpringCloudAdaptiveExt implements AdaptiveExt {
@Override
public String echo(String msg, URL url) {
return "spring cloud";
}
}
// 单元测试3中加上@Adaptive注解,其余不加
@Adaptive
public class ThriftAdaptiveExt implements AdaptiveExt {
@Override
public String echo(String msg, URL url) {
return "thrift";
}
}同时应当在resources目录下新建META-INF/dubbo文件夹,新建com.alibaba.dubbo.demo.provider.adaptive.AdaptiveExt,即接口的全限定名文件,文件内容为:
dubbo=com.alibaba.dubbo.demo.provider.adaptive.impl.DubboAdaptiveExt cloud=com.alibaba.dubbo.demo.provider.adaptive.impl.SpringCloudAdaptiveExt thrift=com.alibaba.dubbo.demo.provider.adaptive.impl.ThriftAdaptiveExt
下面是4个单元测试用例。观察4个测试用例的输出结果,我们可以得出以下结论:
从测试3可以看出,若实现类加了@Adaptive注解,则它优先级最高,getAdaptiveExtension()创建的就是该类的实例
从测试1看出,若SP注解上有值,且url参数中无值,并且没有类标注@Adaptive注解,则创建dubbo的key对应的类的实例
从测试4看出,若方法上有注解@Adpative({"t"}),则URL中应当配上该参数t=cloud,创建cloud对应的实例
从测试2看出,方法有注解@Adaptive,同时URL配置的是默认参数,该参数时通过AdaptiveExt通过转小写生成(adaptive.ext=cloud),则创建的就是cloud对应类的实例,可以看出,其实测试2和4类似,只要URL中有参数并且配置正确,则忽略@SPI注解上的值
所以可以得出优先级: @Adaptive标注的类 > URL参数 > @SPI注解中的值
/**
* SPI上有注解,@SPI("dubbo"),url无参数,没有类上添加@Adaptive注解,方法@Adaptive注解上无参数,输出dubbo
*/
@Test
public void test1(){
ExtensionLoader loader = ExtensionLoader.getExtensionLoader(AdaptiveExt.class);
AdaptiveExt adaptiveExtension = loader.getAdaptiveExtension();
URL url = URL.valueOf("test://localhost/test");
System.out.println(adaptiveExtension.echo("d", url));
} /**
* SPI上有注解,@SPI("dubbo"),URL中也有具体的值,输出spring cloud,注意这里对方法标注有@Adaptive注解,但是该注解没有值
*/
@Test
public void test2(){
ExtensionLoader loader = ExtensionLoader.getExtensionLoader(AdaptiveExt.class);
AdaptiveExt adaptiveExtension = loader.getAdaptiveExtension();
URL url = URL.valueOf("test://localhost/test?adaptive.ext=cloud");
System.out.println(adaptiveExtension.echo("d", url));
} /**
* SPI上有注解,@SPI("dubbo"),URL中也有具体的值,ThriftAdaptiveExt实现类上面有@Adaptive注解,输出thrift
*/
@Test
public void test3(){
ExtensionLoader loader = ExtensionLoader.getExtensionLoader(AdaptiveExt.class);
AdaptiveExt adaptiveExtension = loader.getAdaptiveExtension();
URL url = URL.valueOf("test://localhost/test?adaptive.ext=cloud");
System.out.println(adaptiveExtension.echo("d", url));
} /**
* SPI上有注解,@SPI("dubbo"),URL中也有具体的值,接口方法中加上注解@Adaptive({"t"}),各个实现类上面没有
* @Adaptive注解,输出spring cloud
*/
@Test
public void test4(){
ExtensionLoader loader = ExtensionLoader.getExtensionLoader(AdaptiveExt.class);
AdaptiveExt adaptiveExtension = loader.getAdaptiveExtension();
URL url = URL.valueOf("test://localhost/test?t=cloud");
System.out.println(adaptiveExtension.echo("d", url));
} 首先先分析测试用例对应的源码,其余几种情况都差不多,1种情况分析透彻了,其余几种自然就清楚了.
// SPI上有注解,@SPI("dubbo"),url无参数,没有类上添加@Adaptive注解,方法@Adaptive注解上无参数,输出dubbo
@Test
public void test1(){
ExtensionLoader loader = ExtensionLoader.getExtensionLoader(AdaptiveExt.class);
AdaptiveExt adaptiveExtension = loader.getAdaptiveExtension();
URL url = URL.valueOf("test://localhost/test");
System.out.println(adaptiveExtension.echo("d", url));
} public T getAdaptiveExtension() {
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
if (createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
// 创建自适应拓展代理类对象并放入缓存
instance = createAdaptiveExtension();
cachedAdaptiveInstance.set(instance);
} catch (Throwable t) {
// 抛异常
}
}
}
} else {
// 抛异常
}
}
return (T) instance;
}private T createAdaptiveExtension() {
try {
// 分为3步:1是创建自适应拓展代理类Class对象,2是通过反射创建对象,3是给创建的对象按需依赖注入
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
// 抛异常
}
}
private Class getAdaptiveExtensionClass() {
// 从默认目录中加载标注了@SPI注解的实现类
getExtensionClasses();
// 如果有标注了@Adaptive注解实现类,那么cachedAdaptiveClass不为空,直接返回
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
// 创建自适应拓展代理类class文件
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}private Class createAdaptiveExtensionClass() {
// code就是保存了创建的class字符串数据
String code = createAdaptiveExtensionClassCode();
ClassLoader classLoader = findClassLoader();
Compiler compiler = ExtensionLoader.getExtensionLoader(Compiler.class).getAdaptiveExtension();
return compiler.compile(code, classLoader);
}private String createAdaptiveExtensionClassCode() {
// 用来存放生成的代理类class文件
StringBuilder codeBuilder = new StringBuilder();
// 遍历标注有@SPI注解的接口的所有方法,这里分析的是com.alibaba.dubbo.demo.provider.adaptive.AdaptiveExt
Method[] methods = type.getMethods();
// 这些方法中应当致至少有一个方法被@Adaptive注解标注,否则不需要生成自适应代理类,直接抛出异常
boolean hasAdaptiveAnnotation = false;
for (Method m : methods) {
if (m.isAnnotationPresent(Adaptive.class)) {
hasAdaptiveAnnotation = true;
break;
}
}
// no need to generate adaptive class since there's no adaptive method found.
if (!hasAdaptiveAnnotation)
// 抛异常
// 生成包信息,形如package com.alibaba.dubbo.demo.provider.adaptive;
codeBuilder.append("package ").append(type.getPackage().getName()).append(";");
// 生成导包信息,形如import com.alibaba.dubbo.common.extension.ExtensionLoader;
codeBuilder.append("\nimport ").append(ExtensionLoader.class.getName()).append(";");
// 生成类名,形如public class AdaptiveExt$Adaptive
// implements com.alibaba.dubbo.demo.provider.adaptive.AdaptiveExt {
codeBuilder.append("\npublic class ").append(type.getSimpleName()).append("$Adaptive").
append(" implements ").append(type.getCanonicalName()).append(" {");
// 遍历所有方法,为SPI接口的所有方法生成代理方法
for (Method method : methods) {
// 方法返回值、参数、抛出异常
Class rt = method.getReturnType();
Class[] pts = method.getParameterTypes();
Class[] ets = method.getExceptionTypes();
// 获取方法上的Adaptive注解,如果方法上没有该注解,直接为该方法抛出异常
Adaptive adaptiveAnnotation = method.getAnnotation(Adaptive.class);
StringBuilder code = new StringBuilder(512);
if (adaptiveAnnotation == null) {
code.append("throw new UnsupportedOperationException(\"method ")
.append(method.toString()).append(" of interface ")
.append(type.getName()).append(" is not adaptive method!\");");
} else {
// urlTypeIndex用来记录URL这个参数在第几个参数位置上,这里String echo(String msg, URL url);
// 在位置1上
int urlTypeIndex = -1;
for (int i = 0; i < pts.length; ++i) {
if (pts[i].equals(URL.class)) {
urlTypeIndex = i;
break;
}
}
// 找到了URL参数
if (urlTypeIndex != -1) {
// 空指针检查
// s形如:if (arg1 == null) throw new IllegalArgumentException("url == null");
String s = String.format("\nif (arg%d == null)
throw new IllegalArgumentException(\"url == null\");",urlTypeIndex);
code.append(s);
// s形如:com.alibaba.dubbo.common.URL url = arg1;
s = String.format("\n%s url = arg%d;", URL.class.getName(), urlTypeIndex);
code.append(s);
}
// 没找到,暂不分析,TODO
// 获取方法上的Adaptive注解的值,@Adaptive({"t"}),这里是t
String[] value = adaptiveAnnotation.value();
// 如果@Adaptive注解没有值,对应第二种测试情况,从接口名生成从url中获取参数的key,
// key为adaptive.ext,获取参数为url.getParameter("adaptive.ext", "dubbo")
// 因为第二种情况URL中传递了adaptive.ext这个参数,
// 所以String extName = url.getParameter("t", "dubbo");中获取的是cloud
if (value.length == 0) {
char[] charArray = type.getSimpleName().toCharArray();
StringBuilder sb = new StringBuilder(128);
for (int i = 0; i < charArray.length; i++) {
if (Character.isUpperCase(charArray[i])) {
if (i != 0) {
sb.append(".");
}
sb.append(Character.toLowerCase(charArray[i]));
} else {
sb.append(charArray[i]);
}
}
value = new String[]{sb.toString()};
}
// hasInvocation 暂不分析,TODO
// defaultExtName是dubbo,cachedDefaultName = names[0],这个值是@SPI("dubbo")里的
String defaultExtName = cachedDefaultName;
String getNameCode = null;
for (int i = value.length - 1; i >= 0; --i) {
if (i == value.length - 1) {
if (null != defaultExtName) {
if (!"protocol".equals(value[i]))
if (hasInvocation)
getNameCode =
String.format("url.getMethodParameter(methodName,
\"%s\", \"%s\")", value[i], defaultExtName);
else
// 形如:url.getParameter("t", "dubbo");
// 理解就是看url中有没有传t参数,传了就以url中为准,否则就取
// @SPI("dubbo")中的为默认值
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
getNameCode = String.format("url.getParameter(\"%s\", %s)",
value[i], getNameCode);
else
getNameCode = String.format("url.getProtocol() == null ?
(%s) : url.getProtocol()", getNameCode);
}
}
// 形如:String extName = url.getParameter("t", "dubbo");
// 这个extName就是要为@SPI标注的接口生成哪个代理类
code.append("\nString extName = ").append(getNameCode).append(";");
// check extName == null?
// 形如:if (extName == null) throw new IllegalStateException("...");
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);
// AdaptiveExt extension = (AdaptiveExt)
// ExtensionLoader.getExtensionLoader(AdaptiveExt.class).getExtension(extName);
s = String.format("\n%s extension = (% 0) {
codeBuilder.append(", ");
}
codeBuilder.append(pts[i].getCanonicalName());
codeBuilder.append(" ");
codeBuilder.append("arg").append(i);
}
codeBuilder.append(")");
// 异常
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());
}
return codeBuilder.toString();
} 通过这一系列代码,Dubbo就为AdaptiveExt根据@SPI的注解值dubbo生成了一个自适应拓展代理类,类代码如下:
package com.alibaba.dubbo.demo.provider.adaptive;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class AdaptiveExt$Adaptive implements com.alibaba.dubbo.demo.provider.adaptive.AdaptiveExt {
public java.lang.String echo(java.lang.String arg0, com.alibaba.dubbo.common.URL arg1) {
if (arg1 == null)
throw new IllegalArgumentException("url == null");
com.alibaba.dubbo.common.URL url = arg1;
// 核心,通过上面的分析我们知道,并没有配置t参数,所以URL取不到t参数,则以默认值dubbo代替,而dubbo就是
// @SPI注解的值,adaptiveExtension.echo("d", url),执行这句代码时,adaptiveExtension实际上是
// AdaptiveExt$Adaptive的实例对象,因此会走到它的echo方法中
String extName = url.getParameter("t", "dubbo");
if (extName == null)
throw new IllegalStateException("Fail to get extension(AdaptiveExt) name
from url(" + url.toString() + ") use keys([t])");
// 为了排版布局,使用了简写AdaptiveExt.class,但是应当知道这里应当是全限定名
// 这里面根据extName去获取Adaptive实例对象,获取的是dubbo的key对应的DubboAdaptiveExt实例对象
com.alibaba.dubbo.demo.provider.adaptive.AdaptiveExt extension =
(com.alibaba.dubbo.demo.provider.adaptive.AdaptiveExt)
ExtensionLoader.getExtensionLoader(AdaptiveExt.class).
getExtension(extName);
// 所以会走DubboAdaptiveExt的echo方法,输出dubbo
return extension.echo(arg0, arg1);
}
}分析完了测试用例1,再来分析2和4就简单多了,看代码.归纳起来就是,如果方法上配置了@Adaptive,就将接口名转小写(adaptive.ext),去URL中取这个参数对应的值,即url.getParameter("adaptive.ext", "dubbo")的值作为extName,生成的也是extName对应的类.如果方法上配置了@Adaptive({"t"}),则以url.getParameter("t", "dubbo")这种方式去取值作为extName.
// 获取方法上的Adaptive注解的值,@Adaptive({"t"}),这里是t
String[] value = adaptiveAnnotation.value();
// 如果@Adaptive注解没有值,对应第二种测试情况,从接口名生成从url中获取参数的key,key为adaptive.ext,获取参数
// 为url.getParameter("adaptive.ext", "dubbo")
// 因为第二种情况URL中传递了adaptive.ext这个参数,所以String extName = url.getParameter("t", "dubbo");中获取的是cloud
if (value.length == 0) {
char[] charArray = type.getSimpleName().toCharArray();
StringBuilder sb = new StringBuilder(128);
for (int i = 0; i < charArray.length; i++) {
if (Character.isUpperCase(charArray[i])) {
if (i != 0) {
sb.append(".");
}
sb.append(Character.toLowerCase(charArray[i]));
} else {
sb.append(charArray[i]);
}
}
value = new String[]{sb.toString()};
}// defaultExtName是dubbo,cachedDefaultName = names[0],这个值是@SPI("dubbo")里的
String defaultExtName = cachedDefaultName;
String getNameCode = null;
for (int i = value.length - 1; i >= 0; --i) {
if (i == value.length - 1) {
if (null != defaultExtName) {
if (!"protocol".equals(value[i]))
if (hasInvocation)
// 删除部分代码
else
// 形如:url.getParameter("t", "dubbo");
// 理解就是看url中有没有传t参数,传了就以url中为准,否则就取@SPI("dubbo")中的为默认值
getNameCode = String.format("url.getParameter(\"%s\", \"%s\")", value[i], defaultExtName);
else
// 删除部分代码
} else {
// 删除部分代码
}
}
}
// 形如:String extName = url.getParameter("t", "dubbo");
// 这个extName就是要为@SPI标注的接口生成哪个代理类
code.append("\nString extName = ").append(getNameCode).append(";");接下来分析测试用例3,即ThriftAdaptiveExt类上面标注了@Adaptive注解,前面也说过,它的优先级最高,下面看代码.
private Class getAdaptiveExtensionClass() {
// 1.从默认目录中加载标注了@SPI注解的实现类
getExtensionClasses();
// 2.如果有标注了@Adaptive注解实现类,那么cachedAdaptiveClass不为空,直接返回
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
// 3.创建自适应拓展代理类class文件
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}前面我们分析没有类上面标注@Adaptive注解时,dubbo需要根据配置情况为接口生成自适应拓展代理类,也就是上述对应的步骤3代码.但是当有类标注了@Adaptive注解时,情况就不一样了.看上面步骤1getExtensionClasses()会走到下面loadClass方法,当解析到ThriftAdaptiveExt类时,发现它满足clazz.isAnnotationPresent(Adaptive.class)条件,因此cachedAdaptiveClass = clazz被缓存起来,不会再走后面的逻辑.这样当走步骤2时,直接返回cachedAdaptiveClass.那么dubbo为AdaptiveExt接口生成的自适应拓展就是ThriftAdaptiveExt.