javassist简单介绍以及如何使用它来实现lombok的功能

╰半夏微凉° 2021-12-12 08:39 119阅读 0赞

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbGlzaGFkYWE_size_16_color_FFFFFF_t_70][]

最近在学习MyBatis的源码，在阅读的过程中，发现有一个叫做javaassist的工具，查阅了一些资料，这个工具和ASM的功能类似，可以直接修改Java的字节码文件，而ASM要更偏向底层一些。

下面我们通过一个例子来学习其使用方法:

public class Run {
        public static void main(String[] args) {
            ClassPool pool = ClassPool.getDefault();
            try {
                CtClass ctClass = pool.getCtClass("test01.Cats");
                CtMethod ctMethod = ctClass.getDeclaredMethod("getName");
                for (CtField ctField : ctClass.getDeclaredFields()) {
                    System.out.println(ctField.getName());
                }
                ctMethod.insertBefore("System.out.println(\"teter==est\");");
                ctClass.writeFile();
                Class clazz = ctClass.toClass();
                Constructor constructor = clazz.getConstructor(new Class[]{String.class, Integer.class});
                Cats cats = (Cats) constructor.newInstance("tom", 23);
                System.out.println(cats.getName());
            } catch (NotFoundException e) {
                e.printStackTrace();
            } catch (CannotCompileException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } catch (NoSuchMethodException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (InstantiationException e) {
                e.printStackTrace();
            } catch (InvocationTargetException e) {
                e.printStackTrace();
            }
        }
    }

下面是Cats类的结构，很简单，总共就包含两个属性：

public class Cats implements Cloneable {
        private String name;
        private Integer age;
    
       public Cats(String name, Integer age) {
            this.name = name;
            this.age = age;
        }
    
        public Cats() {
        }
    		
        @Override
        protected Object clone() throws CloneNotSupportedException {
            return super.clone();
        }
        getSet()方法...
    }

我们在Cats类的getName()方法前面添加一处打印的语句，然后执行代码查看结果：

![20190710233933885.PNG][]

除了修改已经存在的类，javaassist工具甚至能够直接拼接出一个class文件:

public class Run02 {
        public static void main(String[] args) {
            ClassPool pool = ClassPool.getDefault();
            CtClass ctClass = pool.makeClass("test01.Dog");
            try {
                CtField name = CtField.make("private String name;", ctClass);
                CtField age = CtField.make("private Integer age;", ctClass);
                ctClass.addField(name);
                ctClass.addField(age);
                CtMethod methodSetName = CtMethod.make("public void setName(String name){this.name=name;}", ctClass);
                CtMethod methodSetAge = CtMethod.make("public void setAge(Integer age){this.age=age;}", ctClass);
                CtMethod meGetName = CtMethod.make("public String getName(){return this.name;}", ctClass);
                CtMethod meGetAge = CtMethod.make("public Integer getAge(){return this.age;}", ctClass);
                ctClass.addMethod(methodSetAge);
                ctClass.addMethod(methodSetName);
                ctClass.addMethod(meGetAge);
                ctClass.addMethod(meGetName);
                CtConstructor ctConstructor = new CtConstructor(new CtClass[]{}, ctClass);
                ctConstructor.setBody("{}");
                CtConstructor ctConstructorWithParam = new CtConstructor(new CtClass[]{pool.get("java.lang.Integer"), pool.get("java.lang.String")}, ctClass);
                ctConstructorWithParam.setBody("{this.name=$2;this.age=$1;}");
                ctClass.addConstructor(ctConstructor);
                ctClass.addConstructor(ctConstructorWithParam);
                ctClass.writeFile();
                //调用反射
                for (CtField ctField : ctClass.getDeclaredFields()) {
                    System.out.println(ctField.getName());
                }
            } catch (CannotCompileException e) {
                e.printStackTrace();
            } catch (NotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

，自此，相信大家对于javaassist这个强大的工具有了一定的了解；于此同时，我又联想到在日常的开发过程中常用的lombok工具，我们可以通过注解的方式来生成一些列的get，set，构造器等，而无需自己手动编写，那它是如何实现的呢？由于本人暂时还没有阅读过lombok的源码，所以猜测也是通过修改字节码的方式对实体类进行了修改，依据这一思路，下面我们借助javaassist来手写一个类似于lombok的小工具吧：

随意创建一个注解:

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface HaHaData {
    
    }

然后在Pay类上边加上对应的注解：

@HaHaData
    public class Pay {
        private String payId;
        private Long userId;
    }

接下来修改class文件：

public class Run03 {
        private static String classLoadPath = "utilTest41.Pay";
    
        public static void main(String[] args) {
            try {
                Class clazz = Class.forName(classLoadPath);
                HaHaData haHaData = (HaHaData) clazz.getAnnotation(HaHaData.class);
                if (haHaData != null) {
                    ClassPool pool = ClassPool.getDefault();
                    CtClass ctClass = pool.getCtClass(classLoadPath);
                    Field[] fields = clazz.getDeclaredFields();
                    CtClass[] ctClasses = new CtClass[fields.length];
                    StringBuilder constructorBody = new StringBuilder();
                    int position = 0;
                    constructorBody.append("{");
                    for (Field field : fields) {
                        StringBuilder sb = new StringBuilder();
                        sb.append("public void set").append(toUpperFristChar(field.getName())).append("(").append(field.getType().getSimpleName()).append(" haHaParam").
                                append(")").append("{").append("this.").append(field.getName()).
                                append("=").append(field.getName()).append(";}");
                        CtMethod methodSet = CtMethod.make(sb.toString(), ctClass);
                        sb.setLength(0);
                        sb.append("public ").append(field.getType().getSimpleName()).append(" get").append(toUpperFristChar(field.getName())).append("()").append("{").append("return this.").append(field.getName()).append(";}");
                        CtMethod methodGet = CtMethod.make(sb.toString(), ctClass);
                        ctClasses[position] = pool.getCtClass(field.getType().getName());
                        constructorBody.append("this.").append(field.getName()).append("=").append("$").append(position + 1).append(";");
                        ctClass.addMethod(methodSet);
                        ctClass.addMethod(methodGet);
                        ++position;
                    }
                    constructorBody.append("}");
                    CtConstructor ctConstructorWithParam = new CtConstructor(ctClasses, ctClass);
                    ctConstructorWithParam.setBody(constructorBody.toString());
                    ctClass.addConstructor(ctConstructorWithParam);
                    ctClass.writeFile("out/production/javaproj/");
                    Class c = Class.forName("utilTest41.Pay");
                    Constructor[] constructors = c.getConstructors();
                    for (Constructor constructor : constructors) {
                        System.out.println(constructor.getName());
                    }
                    Constructor con = c.getDeclaredConstructor(String.class, Long.class);
                }
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (CannotCompileException e) {
                e.printStackTrace();
            } catch (NotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } catch (NoSuchMethodException e) {
                e.printStackTrace();
            }
        }
    
        private static String toUpperFristChar(String string) {
            char[] charArray = string.toCharArray();
            charArray[0] -= 32;
            return String.valueOf(charArray);
        }
    }

最后执行这个修改过的文件:

public class Run04 {
        public static void main(String[] args) {
            Class c = null;
            try {
                c = Class.forName("utilTest41.Pay");
                Constructor[] constructors = c.getConstructors();
                for (Constructor constructor : constructors) {
                    System.out.println(constructor.getName());
                }
                Constructor con = c.getDeclaredConstructor(String.class, Long.class);
                Pay pay = (Pay) con.newInstance("21321321", Long.valueOf(12));
                System.out.println(pay.getPayId());
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (NoSuchMethodException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (InstantiationException e) {
                e.printStackTrace();
            } catch (InvocationTargetException e) {
                e.printStackTrace();
            }
    
        }
    }

getPayId()方法会标红，但是这并不影响执行，我们直接运行查看结果：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbGlzaGFkYWE_size_16_color_FFFFFF_t_70 1][]

最后，如果我们不对jar包做一些加密以及混淆的工作，其实很有可能会被一些人做一些修改，威胁我们的系统，所以我们很有必要对产品做一些加密的工作，当然，就算加密了其实也可以通过dump 内存的方式来进行分析，不过破解的难度会有所提升。

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[20190710233933885.PNG]: /images/20211211/2bb2b61e016849d7ba547d5f6eb8785e.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbGlzaGFkYWE_size_16_color_FFFFFF_t_70 1]: /images/20211211/8a6e91f31785496aa13192cdd8ec679b.png