动态编译（JavaCompiler）

public class Demo01 {
    public static void main(String[] args) {
        JavaCompiler compiler= ToolProvider.getSystemJavaCompiler();
        int result=compiler.run(null, null, null,"d:/myC/HelloWorld.java");
        System.out.println(result==0?"编译成功":"编译失败");


        String str="public class HelloWorld{\n" +
                "\tpublic static void main(String[] args){\n" +
                "\t\tSystem.out.println(\"helloworld\");\n" +
                "\t}\n" +
                "}";

    }
}

public class Demo01 {
    public static void main(String[] args) throws IOException {
        //JavaCompiler编译类
        JavaCompiler compiler= ToolProvider.getSystemJavaCompiler();
        int result=compiler.run(null, null, null,"d:/myC/HelloWorld.java");
        System.out.println(result==0?"编译成功":"编译失败");


        String str="public class HelloWorld{\n" +
                "\tpublic static void main(String[] args){\n" +
                "\t\tSystem.out.println(\"helloworld\");\n" +
                "\t}\n" +
                "}";
        final File htmlFile = File.createTempFile("tmp", ".java");//创建临时文件
        System.out.println("临时文件所在的本地路径：" + htmlFile.getCanonicalPath());
        BufferedWriter bw = new BufferedWriter(new FileWriter(htmlFile));
        result=compiler.run(null, null, null,htmlFile.getCanonicalPath());
        System.out.println(result==0?"编译成功":"编译失败");
        try {
            bw.write(str);
            bw.flush();
        } finally {
            bw.close();
            htmlFile.deleteOnExit();//程序退出时删除临时文件
        }

        //Runtime调用编译类
        Runtime run=Runtime.getRuntime();
        Process process=run.exec("java -cp d:/myC HelloWorld");
        InputStream in=process.getInputStream();
        BufferedReader bw1=new BufferedReader(new InputStreamReader(in));
        String info="";
        while((info=bw1.readLine())!=null){
            System.out.println(info);
        }
    }
}

public static void runJavaClassByReflect() throws Exception{
        try {
            URL[] urls = new URL[] {new URL("file:/"+"d:/myC/")};
            URLClassLoader loader = new URLClassLoader(urls);
            Class c = loader.loadClass("HelloWorld");
            //调用加载类的main方法
            c.getMethod("main",String[].class).invoke(null, (Object)new String[]{});
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

字节码操作：

/**
 * 测试javassist的类
 */
public class Demo01 {
    public static void main(String[] args) throws CannotCompileException, NotFoundException, IOException {
        ClassPool pool=ClassPool.getDefault();
        CtClass cc=pool.makeClass("ssist.Emp");
        //创建属性
        CtField f1=CtField.make("private int empNum;", cc);
        CtField f2=CtField.make("private String ename;", cc);
        cc.addField(f1);
        cc.addField(f2);
        //创建方法
        CtMethod m1=CtMethod.make("public int getEmpNum(){return empNum;}", cc);
        CtMethod m2=CtMethod.make("public void setEmpNum(int empNum){this.empNum=empNum;};", cc);
        cc.addMethod(m1);
        cc.addMethod(m2);
        //添加构造器

        CtConstructor constructor=new CtConstructor(new CtClass[]{CtClass.intType,pool.get("java.lang.String")}, cc);
        constructor.setBody("{this.empNum=empNum;this.ename=ename;}");
        cc.addConstructor(constructor);
        //写入到文件
        cc.writeFile("d:/myJava");
    }
}

/**
 * 测试api
 */
public class Demo02 {
    public static void test() throws NotFoundException, IOException, CannotCompileException {
        ClassPool pool=ClassPool.getDefault();
        CtClass cc=pool.get("ssist.Emp");
        byte[] bytes=cc.toBytecode();
        System.out.println(Arrays.toString(bytes));
        System.out.println(cc.getName());
        //获取简要类名
        System.out.println(cc.getSimpleName());
        //获取父类
        System.out.println(cc.getSuperclass());
        //获取接口
        System.out.println(cc.getInterfaces());
    }

    /**
     * 测试产生新的方法
     */
    public static void tets02() throws CannotCompileException, IllegalAccessException, InstantiationException, NotFoundException, InvocationTargetException, NoSuchMethodException {
        ClassPool pool=ClassPool.getDefault();
        CtClass cc=pool.get("ssist.Emp");

//        CtMethod m=CtNewMethod.make("public int add(int a,int b){return a+b;}",cc);
        CtMethod m=new CtMethod(CtClass.intType, "add", new CtClass[]{CtClass.intType,CtClass.intType},cc);
        m.setModifiers(Modifier.PUBLIC);
        m.setBody("{System.out.println(\"lsd is a good boy\");return $1+$2;}");
        cc.addMethod(m);

        //通过反射调用刚新生成的方法
        Class clazz=cc.toClass();
        Object obj=clazz.newInstance();
        Method m1=clazz.getDeclaredMethod("add",int.class,int.class);
        Object result=m1.invoke(obj,10,20);
        System.out.println(result);
    }

    /**
     * 修改属性信息
     * @throws NotFoundException
     * @throws CannotCompileException
     * @throws IllegalAccessException
     * @throws InstantiationException
     * @throws NoSuchMethodException
     * @throws InvocationTargetException
     */
    public static void test03() throws NotFoundException, CannotCompileException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
        ClassPool pool=ClassPool.getDefault();
        CtClass cc=pool.get("ssist.Emp");

        CtMethod m=cc.getDeclaredMethod("sayHello",  new CtClass[]{CtClass.intType});
        //修改方法前面的内容
        m.insertBefore("System.out.println($1);System.out.println(\"start\");");
        m.insertAt(8, "int b=2;System.out.println(b);");
        //修改方法后面的内容
        m.insertAfter("System.out.println(\"end\");");

        Class clazz=cc.toClass();
        Object obj=clazz.newInstance();
        Method m1=clazz.getDeclaredMethod("sayHello",int.class);
        m1.invoke(obj,10);
    }

    /**
     * 添加属性
     * @throws Exception
     */
    public static void test04() throws Exception{
        ClassPool pool=ClassPool.getDefault();
        CtClass cc=pool.get("ssist.Emp");
        //设置属性
        CtField f1=new CtField(CtClass.intType, "salary",cc);
        f1.setModifiers(Modifier.PRIVATE);
        cc.addField(f1);
        //添加方法
        cc.addMethod(CtNewMethod.getter("getSalart", f1));
        cc.addMethod(CtNewMethod.setter("SetSalart", f1));
    }
    //构造器
    public static void test5() throws Exception {
        ClassPool pool=ClassPool.getDefault();
        CtClass cc=pool.get("ssist.Emp");
        CtConstructor[] constructors=cc.getConstructors();
        for(CtConstructor constructor:constructors){
            System.out.println(constructor);
        }
    }

    public static void main(String[] args) throws Exception {
        test5();
    }
}