Java中的注解和反射实例详解

@Target (ElementType.METHOD) //作用在方法上

@Retention (RetentionPolicy.RUNTIME) //运行时有效

@interface Myannotation1{

  //注解的参数：参数类型 + 参数名();

  String Parameter1() ;

  int Parameter2() default - 1 ; //默认值为-1代表不存在

}

@Target (ElementType.METHOD) //作用在方法上

@Retention (RetentionPolicy.SOURCE ) //源码中有效

@interface Myannotation2{

  String Value() ; //只有一个参数时，参数名用Value

}

//有默认值的参数可以不填没有默认值的参数必填，并且参数填写没有顺序

@Myannotation1 (Parameter1 = "Parameter1 " )

//参数定义为Value时，前面的"Value = "可以省略

@Myannotation2 ( "Value" )

public void test01(){

}

//新建Person类

class Person{

     String name;

     int age;

     int id;

  public Person() {

  }

  public Person(String name, int age) {

  this .name = name;

  this .age = age;

  }

  public String getName() {

  return name;

  }

  public void setName(String name) {

  this .name = name;

  }

  public int getAge() {

  return age;

  }

  public void setAge( int age) {

  this .age = age;

  }

}

//测试

public class test02 {

  public static void main(String[] args) throws ClassNotFoundException {

  Person person = new Person();

  //方式一、通过包名获取

         Class class1 = Class.forName( "com.reflection.Person" );

         System.out.println(class1.hashCode());

         //方式二、通过对象获取

         Class class2 = person.getClass();

         System.out.println(class3.hashCode());

         //方式三、通过类获取

         Class class3 = Person. class ;

         System.out.println(class3.hashCode());

     }

}

public class test03 {

  public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {

  Class c1 = Class.forName( "com.reflection.Person" );

  //获得类的名字

  String name = c1.getName(); //获得包名加类名

  String simpleName = c1.getSimpleName(); //获得类名

  //获得类的属性

  Field[] fields = c1.getFields(); //获得类的所有公有属性

  Field[] declaredFields = c1.getDeclaredFields(); //获得类的所有属性

  Field field = c1.getField(name); //获得类的指定公有属性

  Field declaredField = c1.getDeclaredField(name); //获得类的指定属性（所有类型均可）

  //获得类的方法

  Method[] method = c1.getMethods(); //获得本类及其父类的所有公有方法

  Method[] declaredMethods = c1.getDeclaredMethods(); //获得本类的所有方法

  Method getName = c1.getMethod( "getName" , null ); //获取指定方法(本类及其父类的所有公有方法)

  Method setName = c1.getDeclaredMethod( "setName" , String. class ); //获取指定方法(本类的所有方法)

  //获得类的构造器

  Constructor[] constructors = c1.getConstructors(); //获取所有公有构造器

  Constructor[] declaredConstructors = c1.getDeclaredConstructors(); //获取所有构造器

  Constructor constructor = c1.getConstructor(String. class , int . class ); //获取指定公有构造器

  Constructor declaredConstructor = c1.getDeclaredConstructor(String. class , int . class ); //获取指定构造器

  }

}

public class Test04 {

  public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {

   //获得Class对象

   Class c1 = Class.forName( "com.reflection.Person" );

   //通过反射创建对象

   Person person = (Person)c1.newInstance(); //本质上是调用了无参构造器

   Constructor constructor = c1.getDeclaredConstructor(String. class , int . class ); //获取有参构造器

   Person person2 = (Person) constructor.newInstance( "name" , 18 ); //调用有参构造器创建对象

   //通过反射调用普通方法

   Method setName = c1.getDeclaredMethod( "setName" , String. class ); //获取方法

   setName.invoke(person, "name" ); //通过invoke调用方法

   //通过反射操作属性

   Field name = c1.getDeclaredField( "name" ); //获取name属性

   name.setAccessible( true ); //关闭安全检测，关闭后可以访问私有属性（true为关闭，false为打开，默认是false）

   name.set(person, "lalala" ); //设置属性的值

  }

}

@Testclass ( "db_Student2" ) //类注解

//创建学生类

class Student2{

  @Testfield (columnname = "name" ,type = "varchar" ,length = 20 ) //属性注解

  String name;

  @Testfield (columnname = "age" ,type = "int" ,length = 3 )

  int age;

  @Testfield (columnname = "ID" ,type = "int" ,length = 10 )

  int ID;

  public Student2() {

  }

  public Student2(String name, int age, int ID) {

   this .name = name;

   this .age = age;

   this .ID = ID;

  }

  public String getName() {

   return name;

  }

  public void setName(String name) {

   this .name = name;

  }

  public int getAge() {

   return age;

  }

  public void setAge( int age) {

   this .age = age;

  }

  public int getID() {

   return ID;

  }

  public void setID( int ID) {

   this .ID = ID;

  }

}

//类的注解

@Target (ElementType.TYPE)

@Retention (RetentionPolicy.RUNTIME)

@interface Testclass{

  String value();

}

//属性的注解

@Target (ElementType.FIELD)

@Retention (RetentionPolicy.RUNTIME)

@interface Testfield{

  String columnname();

  String type();

  int length();

}

public class test05 {

  public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {

   Class c1 = Class.forName( "com.zjrcu.reflection.Student2" );

   //通过反射获得注解

   Annotation[] annotations = c1.getAnnotations();

   for (Annotation annotation : annotations) {

    System.out.println(annotation);

   }

   //通过反射获得注解value的值

   Testclass testclass = (Testclass)c1.getAnnotation(Testclass. class );

   String value = testclass.value();

   System.out.println(value);

   //获取类中指定注解的value值

   Field f = c1.getDeclaredField( "name" );

   Testfield testfield = (Testfield) f.getAnnotation(Testfield. class );

   System.out.println(testfield.columnname());

   System.out.println(testfield.length());

   System.out.println(testfield.type());

  }

}