反射学习小结 (初学者自学总结) 学习理由: 框架底层重要的实现原理之一(反射)
学习理由:
框架底层重要的实现原理之一(反射)
反射就是在"java运行系统"时期 到"类加载器验证字节码的正确性"
即运行期间动态的操作类加载器的字节码文件从而获得整个类文件。
1.获取一个类的class对象的方式
eg:获取Person实体类
①:通过.class获取:System.out.println(Person.class);
②:通过class对象中forName()获取:Class<？> aClass = Class.forName("deep.entity.Person");
③:通过类的实例获取:Person person = new Person(); System.out.println(person.getClass());
2、主要方法使用练习
我们有一个BookEntity实体类，通过反射来加载这个类
我封装了四个方法,分别打印类信息、属性信息、方法信息、构造器信息
①:获取类的信息:
1 private static void getClassInfo(){ 2 3 //获取class对象 4 5 Class<BookEntity> clazz = BookEntity.class; 6 7 System.out.println("类的全名称:" + clazz.getName()); 8 9 System.out.println("类的简单类名:" + clazz.getSimpleName()); 10 11 System.out.println("类的修饰符:" + Modifier.toString(clazz.getModifiers())); 12 13 }
②:获取类中所有属性的信息:
private static void getClassFieldInfo() { Class<BookEntity> clazz = BookEntity.class; Object instance = clazz.newIntance(); Field bookNameField = clazz.getDeclareField("bookName"); //bookName字段是私有的需要设置字段访问权限 bookNameField.setAccessible(true); //调用字段对应set方法 bookNameField.set(instance,"java"); //获取刚刚设置到字段中的值 Object value = bookNameField.get(instance); System.out.println(value); }
附上:set方法的源码/get方法的源码
1 @CallerSensitive 2 public void set(Object obj, Object value) 3 throws IllegalArgumentException, IllegalAccessException 4 { 5 if (!override) { 6 if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) { 7 Class<?> caller = Reflection.getCallerClass(); 8 checkAccess(caller, clazz, obj, modifiers); 9 } 10 } 11 getFieldAccessor(obj).set(obj, value); 12 }
1 @CallerSensitive 2 public Object get(Object obj) 3 throws IllegalArgumentException, IllegalAccessException 4 { 5 if (!override) { 6 if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) { 7 Class<?> caller = Reflection.getCallerClass(); 8 checkAccess(caller, clazz, obj, modifiers); 9 } 10 } 11 return getFieldAccessor(obj).get(obj); 12 } 13 14
③:获取方法的所有信息:
1 /** 2 * 获取方法的所有信息 3 * @throws SecurityException 4 * @throws NoSuchMethodException 5 * @throws IllegalAccessException 6 * @throws InstantiationException 7 * @throws InvocationTargetException 8 * @throws IllegalArgumentException 9 */ 10 private static void getClassForMethodInfo() throws NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException { 11 // TODO Auto-generated method stub 12 13 //获取Class对象 14 Class<BookEntity> clazz = BookEntity.class; 15 16 //获取所有public修饰的方法 17 Method[] methods = clazz.getMethods(); 18 for(Method method : methods){ 19 System.out.println(method); 20 System.out.println("访问修饰符:" + Modifier.toString(method.getModifiers())); 21 System.out.println("返回类型名称:" + method.getReturnType().getSimpleName()); 22 System.out.println("方法名称:" + method.getName()); 23 System.out.println("参数类型:" + Arrays.toString(method.getParameterTypes())); 24 System.out.println("参数个数:" + method.getParameterCount() ); 25 System.out.println("----------------------------------------------"); 26 } 27 28 //通过反射实例化一个对象 29 BookEntity instance = clazz.newInstance(); 30 31 //获取单个方法 32 Method set = clazz.getDeclaredMethod("setBookPrice",Double.class); 33 //开启方法访问权限(如果访问修饰符非public) 34 set.invoke(instance,45.7); 35 36 //再获取get方法来获取刚刚设置的值 37 Method get = clazz.getDeclaredMethod("getBookPrice"); 38 39 System.out.println(get.invoke(instance)); 40 41 42 //获取当前类中所有定义的方法 43 methods = clazz.getDeclaredMethods(); 44 for(Method method : methods){ 45 System.out.println(method); 46 } 47 }
1 public java.lang.String deep.entity.BookEntity.getBookName() 2 访问修饰符:public 3 返回类型名称:String 4 方法名称:getBookName 5 参数类型:[] 6 参数个数:0 7 ---------------------------------------------- 8 public java.lang.String deep.entity.BookEntity.getBookAuthor() 9 访问修饰符:public 10 返回类型名称:String 11 方法名称:getBookAuthor 12 参数类型:[] 13 参数个数:0 14 ---------------------------------------------- 15 public java.lang.Integer deep.entity.BookEntity.getBookId() 16 访问修饰符:public 17 返回类型名称:Integer 18 方法名称:getBookId 19 参数类型:[] 20 参数个数:0 21 ---------------------------------------------- 22 public java.lang.Double deep.entity.BookEntity.getBookPrice() 23 访问修饰符:public 24 返回类型名称:Double 25 方法名称:getBookPrice 26 参数类型:[] 27 参数个数:0 28 ---------------------------------------------- 29 public void deep.entity.BookEntity.setBookPrice(java.lang.Double) 30 访问修饰符:public 31 返回类型名称:void 32 方法名称:setBookPrice 33 参数类型:[class java.lang.Double] 34 参数个数:1 35 ---------------------------------------------- 36 public void deep.entity.BookEntity.setBookAuthor(java.lang.String) 37 访问修饰符:public 38 返回类型名称:void 39 方法名称:setBookAuthor 40 参数类型:[class java.lang.String] 41 参数个数:1 42 ---------------------------------------------- 43 public void deep.entity.BookEntity.setBookName(java.lang.String) 44 访问修饰符:public 45 返回类型名称:void 46 方法名称:setBookName 47 参数类型:[class java.lang.String] 48 参数个数:1 49 ---------------------------------------------- 50 public void deep.entity.BookEntity.setBookId(java.lang.Integer) 51 访问修饰符:public 52 返回类型名称:void 53 方法名称:setBookId 54 参数类型:[class java.lang.Integer] 55 参数个数:1 56 ---------------------------------------------- 57 public final void java.lang.Object.wait() throws java.lang.InterruptedException 58 访问修饰符:public final 59 返回类型名称:void 60 方法名称:wait 61 参数类型:[] 62 参数个数:0 63 ---------------------------------------------- 64 public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException 65 访问修饰符:public final 66 返回类型名称:void 67 方法名称:wait 68 参数类型:[long, int] 69 参数个数:2 70 ---------------------------------------------- 71 public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException 72 访问修饰符:public final native 73 返回类型名称:void 74 方法名称:wait 75 参数类型:[long] 76 参数个数:1 77 ---------------------------------------------- 78 public boolean java.lang.Object.equals(java.lang.Object) 79 访问修饰符:public 80 返回类型名称:boolean 81 方法名称:equals 82 参数类型:[class java.lang.Object] 83 参数个数:1 84 ---------------------------------------------- 85 public java.lang.String java.lang.Object.toString() 86 访问修饰符:public 87 返回类型名称:String 88 方法名称:toString 89 参数类型:[] 90 参数个数:0 91 ---------------------------------------------- 92 public native int java.lang.Object.hashCode() 93 访问修饰符:public native 94 返回类型名称:int 95 方法名称:hashCode 96 参数类型:[] 97 参数个数:0 98 ---------------------------------------------- 99 public final native java.lang.Class java.lang.Object.getClass() 100 访问修饰符:public final native 101 返回类型名称:Class 102 方法名称:getClass 103 参数类型:[] 104 参数个数:0 105 ---------------------------------------------- 106 public final native void java.lang.Object.notify() 107 访问修饰符:public final native 108 返回类型名称:void 109 方法名称:notify 110 参数类型:[] 111 参数个数:0 112 ---------------------------------------------- 113 public final native void java.lang.Object.notifyAll() 114 访问修饰符:public final native 115 返回类型名称:void 116 方法名称:notifyAll 117 参数类型:[] 118 参数个数:0 119 ---------------------------------------------- 120 45.7 121 public java.lang.String deep.entity.BookEntity.getBookName() 122 public java.lang.String deep.entity.BookEntity.getBookAuthor() 123 public java.lang.Integer deep.entity.BookEntity.getBookId() 124 public java.lang.Double deep.entity.BookEntity.getBookPrice() 125 public void deep.entity.BookEntity.setBookPrice(java.lang.Double) 126 public void deep.entity.BookEntity.setBookAuthor(java.lang.String) 127 public void deep.entity.BookEntity.setBookName(java.lang.String) 128 public void deep.entity.BookEntity.setBookId(java.lang.Integer)
④获取构造器信息:
1 /** 2 * 获取构造器信息 3 * @throws SecurityException 4 * @throws NoSuchMethodException 5 * @throws InvocationTargetException 6 * @throws IllegalArgumentException 7 * @throws IllegalAccessException 8 * @throws InstantiationException 9 */ 10 private static void getClassForConstructor() throws NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException { 11 // TODO Auto-generated method stub 12 //获取class对象 13 Class<BookEntity> clazz = BookEntity.class; 14 //获取所有public修饰的函数 15 Constructor<?>[] constructors = clazz.getConstructors(); 16 17 for(Constructor constructor : constructors){ 18 System.out.println(constructor); 19 } 20 21 System.out.println("------------------------------------------"); 22 23 //湖区类中所有定义的构造函数 24 constructors = clazz.getDeclaredConstructors(); 25 for(Constructor constructor : constructors){ 26 System.out.println(constructor); 27 System.out.println("访问修饰符:" + Modifier.toString(constructor.getModifiers())); 28 System.out.println("构造器名称:" + constructor.getName()); 29 System.out.println("参数列表:" + Arrays.toString(constructor.getParameterTypes())); 30 System.out.println("----------------------------------------"); 31 } 32 33 //调用指定构造函数 34 Constructor<BookEntity> constructor = clazz.getDeclaredConstructor(int.class,String.class); 35 constructor.setAccessible(true); 36 BookEntity instance = constructor.newInstance(100,"三国演义"); 37 System.out.println(instance); 38 System.out.println(instance.getBookName()); 39 System.out.println(instance.getBookAuthor()); 40 System.out.println(instance.getBookId()); 41 System.out.println(instance.getBookPrice()); 42 }