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注解的用处之一就是自动生成包含程序额外信息的"附文件"。Java EE 5使用注解极大地简化了编程模型。

javac -processor ProcessorClassName1,ProcessorClassName2,... sourceFiles

有编译器定位源代码中的注解，然后选择恰当的注解处理器。每个注解处理器会依次执行。如果某哥哥注解处理器创建了一个新的源文件，那么将重复执行这个处理过程。如果某次处理循环没有再产生任何新的源文件，那么就编译所有的源文件。

@SupportedAnnotationTypes("annotation.Property")    @SupportedSourceVersion(SourceVersion.RELEASE_6)

通过一个简单示例来演示这项技术，一个DEMO，可以自动产生bean信息类。可以使用一个注解来标记bean的属性，然后运行某个工具对这个源文件进行解析、分析其注解，最后输出bean信息类的源文件。

package com.shaogq.annotation.source;import java.beans.Introspector;import java.io.IOException;import java.io.PrintWriter;import java.util.LinkedHashMap;import java.util.Map;import java.util.Set;import javax.annotation.processing.AbstractProcessor;import javax.annotation.processing.RoundEnvironment;import javax.annotation.processing.SupportedAnnotationTypes;import javax.annotation.processing.SupportedSourceVersion;import javax.lang.model.SourceVersion;import javax.lang.model.element.Element;import javax.lang.model.element.TypeElement;import javax.tools.Diagnostic.Kind;import javax.tools.JavaFileObject;@SupportedAnnotationTypes("com.shaogq.annotation.Property")@SupportedSourceVersion(SourceVersion.RELEASE_6)public class BeanInfoAnnotationFactory extends AbstractProcessor{    @Override    public boolean process(Set
    annotations,            RoundEnvironment roundEnv) {        for(TypeElement t : annotations){            Map
   
     props = new LinkedHashMap
    
     ();            String beanClassName = null;            for(Element e:roundEnv.getElementsAnnotatedWith(t)){                String mname = e.getSimpleName().toString();                String[] prefixes = {"get", "set" , "is"};                boolean found = false;                for(int i=0;!found && i
     
       props) throws IOException{        JavaFileObject sourceFile = processingEnv.getFiler().createSourceFile(                beanClassName + "BeanInfo");        PrintWriter out = new PrintWriter(sourceFile.openWriter());        int i = beanClassName.lastIndexOf(".");        if(i>0){            out.print("package ");            out.print(beanClassName.substring(0, i));            out.print(";");        }        out.print("public class ");        out.print(beanClassName.substring(i + 1));        out.println("BeanInfo extends java.beans.SimpleBeanInfo");        out.print("{");        out.print("    public java.beans.PropertyDescriptor[] getPropertyDescriptors(){");        out.println("        try{");        for(Map.Entry
      
        e :props.entrySet()){            out.print("            java.beans.PropertyDescriptor");            out.print(e.getKey());            out.println("Descriptor");            out.print("                = new java.beans.PropertyDescriptory(\"");            out.print(e.getKey());            out.print("\", ");            out.print(beanClassName);            out.println(".class);");            String ed = e.getValue().editor().toString();            if(!ed.equals("")){                out.print("            ");                out.print(e.getKey());                out.print("Descriptor.setPropertyEditorClass(");                out.print(ed);                out.println(".class);");            }        }        out.println("           return new java.beans.PropertyDescriptor[]{");        boolean first = true;        for(String p : props.keySet()){            if(first){                first = false;            }else{                out.print(",");            }            out.println();            out.print("        ");            out.print(p);            out.print("Descriptor");        }        out.println();        out.println("            };");        out.println("        }");        out.println("        catch(java.beans.IntrospectionException e){");        out.println("            e.printStackTrace();");        out.println("        return null;");        out.println("        }");        out.println("    }");        out.println("}");        out.close();    }    }
      
     
    
   

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