Java字节码的动态生成和增强

Java字节码的动态生成

     实例

         package  test;

import  java.lang.reflect.Method;

import  java.net.URI;

import  java.net.URISyntaxException;

import  java.util.Arrays;

import  javax.tools.JavaCompiler;

import  javax.tools.JavaCompiler.CompilationTask;

import  javax.tools.SimpleJavaFileObject;

import  javax.tools.StandardJavaFileManager;

import  javax.tools.ToolProvider;

public  class  CompilerTest {

         public  static  void  main(String[] args)  throws  URISyntaxException {

             

               try  {

                      double  value = calculate(  "(1+2)*8-5" );

                    System.  out .println( "value:"  +value);

             }  catch  (Exception e) {

                    e.printStackTrace();

             }

             

//           try {

//                  double value = invokeMethod("CalculatorMain", "calculate");

//                  System.out.println("value:"+value);

//           } catch (Exception e) {

//                  e.printStackTrace();

//           }

       }

         private  static  double  calculate(String expr)  throws  Exception {

             String className =  "CalculatorMain" ;

             String methodName =  "calculate" ;

             

//           String source = "public class Main{" +

//                         "public static void main(String[]  args ){System.out.println(\"Hello World\");}" +

//                         "}";

             

             String source =  "public class " +className+ "{"  +

                             "public static double " +methodName+ "(){return "  + expr+ ";}" +

                             "}" ;

             

               //获取平台的编译器

             JavaCompiler compiler = ToolProvider.  getSystemJavaCompiler ();

             

               //java文件管理器

             StandardJavaFileManager fileManager = compiler.getStandardFileManager( null  ,  null ,  null );

             

               //java源文件对象

             StringSourceJavaObject sourceObject =  new  CompilerTest.StringSourceJavaObject(className, source);

             

             Iterable<StringSourceJavaObject> fileObjects = Arrays. asList (sourceObject);

             

               //Writer writer = new BufferedWriter(new FileWriter(new File("/myclass")));

             

             

             String flag =  "-d" ;

             String  outDir  = System. getProperty ( "user.dir"  ) +  "/"  +  "bin"  ;

             

             System.  out .println( "outDir:"  +outDir );

             

               //设置编译器要执行的操作。-d 设置编译生成的.class放到哪一个目录

             Iterable<String> stringdir = Arrays.  asList (flag, outDir  );

             

             System.  out .println(stringdir);

             

             CompilationTask task = compiler.getTask(  null , fileManager,  null , stringdir,  null , fileObjects);

             

               boolean  result = task.call();

             

               if (result){

                    System.  out .println( "编译成功"  );

                    

                    

                      return  invokeMethod(className, methodName);

             }  else {

                      throw  new  RuntimeException( "错误的表达式" );

             }

             

       }

         private  static  double  invokeMethod(String className, String methodName)

                      throws  Exception {

             ClassLoader classLoader = CompilerTest.  class .getClassLoader();

               try  {

                    Class<?> clazz = classLoader.loadClass(className);

                    Method method = clazz.getMethod(methodName,  new  Class<?>[]{});

                    Object value = method.invoke(  null ,  new  Object[]{});

                      return  (Double)value;

             }  catch  (Exception e) {

                      throw  e;

             }

       }

       

         static  class  StringSourceJavaObject  extends  SimpleJavaFileObject{

               private  String  content  ;

               public  StringSourceJavaObject(String name,String content)  throws  URISyntaxException{

                      super (URI.create(  "String:///" +name.replace( "."  ,  "/" )+Kind. SOURCE  . extension  ),Kind. SOURCE ); //Kind.SOURCE源文件类型

                      this . content  = content;

             }

             

               public  CharSequence getCharContent( boolean  ignoreEncodingErrors){

                      return  content  ;

             }

       }

}

Java字节代码的增强

     

      Java字节代码的增强的时机是在java源代码编译之后，在JVM执行之前

     非常有助于进行性能分析、调试跟踪、日志记录，也可以动态的添加一些因语法而添加的东西，可以有效的帮助开发人员从中解脱出来

     比较有名的工具有     ASM、cglib