java反射

1. 概念

官方解释：反射是指在程序运行中，动态获取类的信息，并且访问

我们通过反射还是获取类的信息（属性，方法，构造方法），然后访问，只不过我们现在不再以new的方式获取

java中反射相关的API都位于java.lang.reflect包下

万物皆对象

关键字：

　　class　　类

　　Method　　　方法

　　Filed　　　　属性

　　Constructor　构造方法

2. 使用反射获取属性

2.1获取public修饰的属性

getFields() 获取所有的public修饰的属性

getField(String naem)根据名字获取单个属性对象

 1 package com.qfedu.test1;
 2 
 3 import java.lang.reflect.Field;
 4 /**
 5  *     获取public声明的所有的属性信息 并且访问
 6  * @author WHD
 7  *
 8  */
 9 public class Student {
10     public String name;
11     public int age;
12     public String address;
13 
14     public Student() {
15         System.out.println("Student类的无参构造");
16     }
17 
18     public static void main(String[] args) {
19         try {
20             // 根据包名+ 类名 (全限定名)获取类信息文件
21             Class<?> stuClass = Class.forName("com.qfedu.test1.Student");
22             // 调用Class类的newInstance方法 构造实例
23             Object newInstance1 = stuClass.newInstance();
24             Object newInstance2 = stuClass.newInstance();
25             Object newInstance3 = stuClass.newInstance();
26 
27             // 通过Class对象获取到当前类所有的属性 返回值为字段数组
28             Field [] fs = stuClass.getFields();
29             for(Field f : fs) {
30                 System.out.println(f.getName());
31             }
32 
33             // 根据字段名称获取到单个字段对象
34             Field nameField = stuClass.getField("name");
35 
36             // 调用字段set方法 设置属性值
37             nameField.set(newInstance2, "赵四");
38 
39             // 调用字段get方法  获取属性值
40             System.out.println(nameField.get(newInstance2));
41 
42             Field ageField = stuClass.getField("age");
43 
44             ageField.set(newInstance1, 20);
45             System.out.println(ageField.get(newInstance1));
46 
47             Field addressField = stuClass.getField("address");
48             addressField.set(newInstance3, "象牙山");
49             System.out.println(addressField.get(newInstance3));
50 
51 
52         } catch (ClassNotFoundException e) {
53             e.printStackTrace();
54         } catch (InstantiationException e) {
55             e.printStackTrace();
56         } catch (IllegalAccessException e) {
57             e.printStackTrace();
58         } catch (NoSuchFieldException e) {
59             e.printStackTrace();
60         } catch (SecurityException e) {
61             e.printStackTrace();
62         }
63     }
64 
65 }

2.2获取已声明的属性

getDeclaredFields()获取所有已定义的属性对象包括 private 默认 Protected public

getDeclaredField(String name)根据名字获取单个属性对象可以是任意访问修饰符

 1 package com.qfedu.test2;
 2 
 3 import java.lang.reflect.Field;
 4 
 5 /**
 6  *     获取到类中所有的属性
 7  * @author WHD
 8  *
 9  */
10 public class Person {
11     private String name;
12     int age;
13     protected String address;
14     public char sex;
15     
16     public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, SecurityException, InstantiationException, IllegalAccessException {
17         Class<?> personClass = Class.forName("com.qfedu.test1.Person");
18         Field[] fs = personClass.getDeclaredFields();
19         for (Field field : fs) {
20             System.out.println(field.getName() + "\t" +field.getType());
21         }
22         
23         Object newInstance = personClass.newInstance();
24         Field nameField = personClass.getDeclaredField("name");
25         nameField.set(newInstance, "广坤");
26         System.out.println(nameField.get(newInstance));
27         
28         Field sexField = personClass.getDeclaredField("sex");
29         sexField.set(newInstance, '男');
30         System.out.println(sexField.get(newInstance));
31             
32     }
33 }

 1 package com.qfedu.test2;
 2 
 3 import java.lang.reflect.Field;
 4 /**
 5  *     获取属性
 6  *     getFields() 获取所有的public修饰的属性
 7  *     getField(String name)根据名字获取单个属性对象
 8  *     getDeclaredFields() 获取所有已定义的属性对象 包括 private 默认 protected public
 9  *     getDeclaredField(String name) 根据名字获取单个属性对象 可以是任意访问修饰符 
10  * @author WHD
11  *
12  */
13 public class TestPerson {
14     public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException, InstantiationException {
15 //        Person p = new Person();
16 //        p.name = ""
17         
18         Class<?> personClass = Class.forName("com.qfedu.test1.Person");
19         
20         Field nameField = personClass.getDeclaredField("name");
21         
22         nameField.setAccessible(true); // 设置访问权限为true 表示忽略JVM非法访问异常
23         
24         
25         
26         System.out.println(nameField.getName() + nameField.getType());
27         Object newInstance = personClass.newInstance();
28         nameField.set(newInstance, "赵四");
29         System.out.println(nameField.get(newInstance));
30     }
31 }

3.使用反射获取方法

3.1获取public修饰的，和继承自父类的方法

getMethods() 获取本类所有public修饰的方法和继承父类的方法

getMethod() 获取单个的方法

 1 package com.qfedu.test3;
 2 
 3 import java.lang.reflect.InvocationTargetException;
 4 import java.lang.reflect.Method;
 5 
 6 public class TestStudent {
 7     public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
 8         Class<?> studentClass = Class.forName("com.qfedu.test3.Student");
 9         Method[] methods = studentClass.getMethods();
10          System.out.println(methods.length);
11         for (Method method : methods) {
12              System.out.println(method.getName());
13         }
14         Object newInstance = studentClass.newInstance();
15         Method m1Method = studentClass.getMethod("m1");
16         
17         m1Method.invoke(newInstance);
18         
19         
20         Method method = studentClass.getMethod("m1", String.class);
21         method.invoke(newInstance, "赵四");
22         
23         
24         Method method2 = studentClass.getMethod("m1", int.class);
25         method2.invoke(newInstance, 330);
26         
27         
28         Method method3 = studentClass.getMethod("m3", String.class,int.class);
29         method3.invoke(newInstance, "赵四",22);
30     }
31 }

3.2获取本类中定义的方法

getDeclaredmethod(String name, Class<?>....parameterTypes)获取单个本类中声明的方法如果访问权限不足可以使用setAccessable(true)修改权限

getDeclaredMethobs()获取所有本类中已经定义的方法

 1 package com.qfedu.test4;
 2 
 3 import java.lang.reflect.InvocationTargetException;
 4 import java.lang.reflect.Method;
 5 
 6 public class TestStudent {
 7     public static void main(String[] args) {
 8         try {
 9             Class<?> stuClass = Class.forName("com.qfedu.test4.Student");
10             Method[] ms = stuClass.getDeclaredMethods();
11             for (Method method : ms) {
12                 System.out.println(method.getName());
13             }
14             Object newInstance = stuClass.newInstance();
15             Method m1 = stuClass.getDeclaredMethod("m1");
16             m1.setAccessible(true); // 私有方法必须设置访问权限为true
17             m1.invoke(newInstance);
18             
19             
20             Method m2 = stuClass.getDeclaredMethod("m1", String.class);
21             m2.invoke(newInstance, "赵四");
22             
23             
24             Method m3 = stuClass.getMethod("m3", String.class,int.class);
25             m3.invoke(newInstance, "广坤",20);
26             
27         } catch (ClassNotFoundException e) {
28             e.printStackTrace();
29         } catch (NoSuchMethodException e) {
30             e.printStackTrace();
31         } catch (SecurityException e) {
32             e.printStackTrace();
33         } catch (IllegalAccessException e) {
34             e.printStackTrace();
35         } catch (IllegalArgumentException e) {
36             e.printStackTrace();
37         } catch (InvocationTargetException e) {
38             e.printStackTrace();
39         } catch (InstantiationException e) {
40             e.printStackTrace();
41         }
42     }
43 }

4.使用反射获取构造器

4.1获取所有public修饰的构造器

getConstructors()获取public修饰的构造方法

getConstructor(class...type)获取单个的构造器对象

 1 package com.qfedu.test5;
 2 
 3 /**
 4  *     获取本类中定义的以public修饰的构造方法
 5  * @author WHD
 6  *
 7  */
 8 public class Student {
 9     public String name;
10     public int age;
11     public String address;
12 
13     public Student() {
14         System.out.println("Student类的无参构造");
15     }
16 
17     public Student(String name, int age, String address) {
18         this.name = name;
19         this.age = age;
20         this.address = address;
21         System.out.println("三个参数的构造方法");
22     }
23 
24     public Student(String name) {
25         this.name = name;
26         System.out.println("一个参数的构造方法");
27     }
28 
29     public Student(String name, int age) {
30         this.name = name;
31         this.age = age;
32         System.out.println("两个参数的构造方法");
33     }
34 }

 1 package com.qfedu.test5;
 2 
 3 import java.lang.reflect.Constructor;
 4 import java.lang.reflect.InvocationTargetException;
 5 
 6 public class TestStudent {
 7     public static void main(String[] args) {
 8         try {
 9             Class<?> stuClass = Class.forName("com.qfedu.test5.Student");
10             Constructor<?>[] cs = stuClass.getConstructors();
11             for (Constructor<?> c : cs) {
12                 System.out.println("名字" + c.getName() + "参数个数" + c.getParameterCount());
13             }
14             
15             Constructor<?> constructor1 = stuClass.getConstructor();
16             Object newInstance1 = constructor1.newInstance();
17             
18             
19             Constructor<?> constructor2 = stuClass.getConstructor(String.class,int.class,String.class);
20             Object newInstance2 = constructor2.newInstance("赵四",20,"象牙山");
21             
22             
23             Constructor<?> constructor3 = stuClass.getConstructor(String.class);
24             Object newInstance3 = constructor3.newInstance("广坤");
25             
26             
27         } catch (ClassNotFoundException e) {
28             e.printStackTrace();
29         } catch (NoSuchMethodException e) {
30             e.printStackTrace();
31         } catch (SecurityException e) {
32             e.printStackTrace();
33         } catch (InstantiationException e) {
34             e.printStackTrace();
35         } catch (IllegalAccessException e) {
36             e.printStackTrace();
37         } catch (IllegalArgumentException e) {
38             e.printStackTrace();
39         } catch (InvocationTargetException e) {
40             e.printStackTrace();
41         }
42     }
43 }

4.2获取本类中定义的构造器

getDeclaredConstructors() 获取本类中定义的所有的构造方法

getDeclaredConstructor() 获取本类中单个的构造方法

 1 package com.qfedu.test6;
 2 
 3 /**
 4  *     获取本类中定义的以public修饰的构造方法
 5  * @author WHD
 6  *
 7  */
 8 public class Student {
 9     public String name;
10     public int age;
11     public String address;
12 
13     private Student() {
14         System.out.println("Student类的无参构造");
15     }
16 
17      Student(String name, int age, String address) {
18         this.name = name;
19         this.age = age;
20         this.address = address;
21         System.out.println("三个参数的构造方法");
22     }
23 
24     protected Student(String name) {
25         this.name = name;
26         System.out.println("一个参数的构造方法");
27     }
28 
29     public Student(String name, int age) {
30         this.name = name;
31         this.age = age;
32         System.out.println("两个参数的构造方法");
33     }
34 }

 1 package com.qfedu.test6;
 2 
 3 import java.lang.reflect.Constructor;
 4 import java.lang.reflect.InvocationTargetException;
 5 
 6 public class TestStudent {
 7     public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
 8             Class<?> stuClass = Class.forName("com.qfedu.test6.Student");
 9             Constructor<?>[] cs = stuClass.getDeclaredConstructors();
10             System.out.println(cs.length);
11             
12             
13             Constructor<?> cons1 = stuClass.getDeclaredConstructor();
14             cons1.setAccessible(true);
15             Object newInstance1 = cons1.newInstance();
16     }
17 }

posted @ 2021-04-21 17:50 青草的骨头阅读(61) 评论(0) 收藏举报

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