javaassist对class的字节码的动态修改

偏执的太偏执、 2023-02-14 01:23 4阅读 0赞

### javaassist的maven依赖： ###

<dependency>
      <groupId>javassist</groupId>
      <artifactId>javassist</artifactId>
      <version>3.12.1.GA</version>
    </dependency>

### Person类： ###

public class Person {
        private String name;
        private int age;
        private boolean sex;
        private String address;
    }

### 动态添加方法： ###

ClassPool classPool = ClassPool.getDefault();
    
            CtClass ctClass = classPool.makeClass("application.Person");
    
            CtMethod ctMethod = CtNewMethod.make("public void sayHello(String name){\n" +
                    "        System.out.println(\"得到的是一个名称\" + name);\n" +
                    "    }",ctClass);
    
            ctClass.addMethod(ctMethod);
    
            Class aClass = ctClass.toClass();
    
            Object object = aClass.newInstance();
            Method sayHello = aClass.getDeclaredMethod("sayHello", String.class);
    
            sayHello.invoke(object,"王天龙");

### 动态添加属性： ###

ClassPool classPool = ClassPool.getDefault();
    
            CtClass ctClass = classPool.get("application.Person");
    
            CtField ctField = CtField.make("private String phone;", ctClass);
            ctClass.addField(ctField);
            Class aClass = ctClass.toClass();
            Field[] declaredFields = aClass.getDeclaredFields();
    
            for (Field field : declaredFields){
                System.out.println(field);
            }

运行结果：

private java.lang.String application.Person.name
    private int application.Person.age
    private boolean application.Person.sex
    private java.lang.String application.Person.address
    private java.lang.String application.Person.phone

### 动态添加构造函数： ###

ClassPool classPool = ClassPool.getDefault();
    
            CtClass ctClass = classPool.get("application.Person");
            CtConstructor ctConstructor = CtNewConstructor.make("public Person(String name, int age, boolean sex, String address) {\n" +
                    "        this.name = name;\n" +
                    "        this.age = age;\n" +
                    "        this.sex = sex;\n" +
                    "        this.address = address;\n" +
                    "    }", ctClass);
            ctClass.addConstructor(ctConstructor);
    
            Class aClass = ctClass.toClass();
            Constructor[] declaredConstructors = aClass.getDeclaredConstructors();
            for (Constructor constructor : declaredConstructors){
                System.out.println(constructor);
            }

运行结果：

public application.Person()
    public application.Person(java.lang.String,int,boolean,java.lang.String)

### 动态添加注解： ###

ClassPool classPool = ClassPool.getDefault();
    
            CtClass ctClass = classPool.get("application.Person");
            ClassFile classFile = ctClass.getClassFile();
            ConstPool constPool = classFile.getConstPool();
    
            AnnotationsAttribute annotationsAttribute = new AnnotationsAttribute(constPool, AnnotationsAttribute.visibleTag);
    
            Annotation annotation = new Annotation("org.springframework.stereotype.Service",constPool);
    
            annotationsAttribute.addAnnotation(annotation);
            classFile.addAttribute(annotationsAttribute);
    
            Class aClass = ctClass.toClass();
            java.lang.annotation.Annotation[] declaredAnnotations = aClass.getDeclaredAnnotations();
            for (java.lang.annotation.Annotation anno : declaredAnnotations){
                System.out.println(anno);
            }

运行结果：

@org.springframework.stereotype.Service(value=)

动态插入代码块：

ClassPool classPool = ClassPool.getDefault();
    
            CtClass ctClass = classPool.get("application.Person");
    
            CtMethod sayHello = ctClass.getDeclaredMethod("sayHello", new CtClass[]{
                    classPool.get(String.class.getCanonicalName())
            });
            sayHello.insertBefore("int num1 = 1;\n" +
                    "        int num2 = 2;\n" +
                    "        System.out.println(num1 + num2);");
            sayHello.insertAt(22,"System.out.println(\"插入的代码\");");
            sayHello.insertAfter("int num1 = 1;\n" +
                    "        int num2 = 2;\n" +
                    "        System.out.println(num1 + num2);");
            Class aClass = ctClass.toClass();
            Object object = aClass.newInstance();
            Method method = aClass.getDeclaredMethod("sayHello", String.class);
    
            method.invoke(object,"王天龙");