Java字节码的动态生成和增强

最新推荐文章于 2025-03-26 22:50:07 发布

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最新推荐文章于 2025-03-26 22:50:07 发布

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分类专栏： Java探险

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本文介绍如何在Java中动态生成字节码，通过一个实例展示了使用JavaCompiler进行编译，并创建StringSourceJavaObject实现源代码的构建。同时讨论了字节码增强的概念，指出其在性能分析、调试和日志记录等方面的应用，并提到了ASM和cglib等知名工具。

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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 ;

             }

       }

}