Java字节码的动态生成和增强

最新推荐文章于 2023-07-16 22:07:24 发布

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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 ;

             }

       }

}