JDK源码/轮子分析 ：反射包 java.lang.reflect③ 之 Field 获取类的属性字段信息

    今天的主角：java.lang.reflect.Field

    继前面说的，一个类有它的抽象，构造方法有它的抽象，当然我们类中的字段也可以有它的抽象。 Field 也就是我们说的字段，类中的属性。一般地我们在类中看到的样子是这样的： private String privateStr ; public static final  String publicStr ;          String protectStr ; public static final String finalStr = "110" ;     从上面可以看出，一个字段对象应该有5部份：字段的权限（private、public、protect）,是否静态是否不可变，字段的类型（String、Date等），字段名，字段值。

获取Field 对象

    Class 对象中获取Field 对象的方法有：      获取所有权限的字段     Field    getDeclaredField(String name)     Field[]    getDeclaredFields()

获取公共权限的字段     Field    getField(String name)     Field[]    getFields()

    首先贴上我们的测试pojo类 package com.hlmtest.java.reflect; public class TestFieldClass {       private String privateStr ;       public   String publicStr ;                 String protectStr ;       public static final String finalStr = "110" ;             public TestFieldClass(){}             public TestFieldClass(String privateStr ,String publicStr ,String protectStr ){             this . privateStr = privateStr ;             this . publicStr = publicStr ;             this . protectStr = protectStr ;                  }       public String getPrivateStr() {             return privateStr ;      }       public void setPrivateStr(String privateStr ) {             this . privateStr = privateStr ;      }       public String getPublicStr() {             return publicStr ;      }       public void setPublicStr(String publicStr ) {             this . publicStr = publicStr ;      }       public String getProtectStr() {             return protectStr ;      }       public void setProtectStr(String protectStr ) {             this . protectStr = protectStr ;      }       public String toString(){             return "privateStr:" + privateStr + ";publicStr:" + publicStr + ";protectStr:" + protectStr + ";finalStr:" + finalStr ;                }             }

package com.hlmtest.java.reflect; import java.lang.reflect.Field; public class TestFieldObject {       public static void main(String[] args ) {             getFieldInfo ();      }             public static void getFieldInfo(){             //先获取Class 对象实例            Class<TestFieldClass> clzz = TestFieldClass. class ;            Field[] DeclFieldList =  clzz .getDeclaredFields();             for (Field field : DeclFieldList ){                 System. out .println( field .toString()+ ";" );            }            System. out .println( "<--我是华丽的分割线-->" );                        Field[] PublFieldList =  clzz .getFields();             for (Field field : PublFieldList ){                 System. out .println( field .toString()+ ";" );            }                  } }     运行结果为： private java.lang.String com.hlmtest.java.reflect.TestFieldClass.privateStr; public java.lang.String com.hlmtest.java.reflect.TestFieldClass.publicStr; java.lang.String com.hlmtest.java.reflect.TestFieldClass.protectStr; public static final java.lang.String com.hlmtest.java.reflect.TestFieldClass.finalStr; <--我是华丽的分割线--> public java.lang.String com.hlmtest.java.reflect.TestFieldClass.publicStr; public static final java.lang.String com.hlmtest.java.reflect.TestFieldClass.finalStr;

取字段值

    根据字段名取得字段 package com.hlmtest.java.reflect; import java.lang.reflect.Field; public class TestFieldObject {       public static void main(String[] args ) throws NoSuchFieldException, SecurityException {             getFieldInfo ();      }             public static void getFieldInfo() throws NoSuchFieldException, SecurityException{             //先获取Class 对象实例            Class<TestFieldClass> clzz = TestFieldClass. class ;            Field field =  clzz .getDeclaredField( "publicStr" );            System. out .println( field .toString()+ ";" );                  }       }     

运行结果： public java.lang.String com.hlmtest.java.reflect.TestFieldClass.publicStr;

通过Field 对象获取某实例的字段值： Object    get(Object obj) import java.lang.reflect.InvocationTargetException; public class TestFieldObject {       public static void main(String[] args ) throws NoSuchFieldException, SecurityException, NoSuchMethodException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {             getFieldInfo ();      }             public static void getFieldInfo() throws NoSuchFieldException, SecurityException, NoSuchMethodException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException{             //先获取Class 对象实例            Class<TestFieldClass> clzz = TestFieldClass. class ;                         //获取有参的构造方法对象             @SuppressWarnings ( "rawtypes" )            Class[] paramstr = {String. class ,String. class ,String. class };            Constructor<TestFieldClass> instance = clzz .getConstructor( paramstr );             //调用Constructor 的newInstance(Object... initargs ) 方法获得主对象实例            TestFieldClass obj = instance .newInstance( "我是私有属性" , "我是公共属性" , "我是默认属性" );                        Field field =  clzz .getDeclaredField( "publicStr" );            Object valueObj = field .get( obj );            System. out .println( valueObj .toString()+ "" );                        Field[] fieldList =  clzz .getFields();             for (Field f : fieldList ){                 Object value = f .get( obj );                 System. out .println( f .toString()+ "-->" + value .toString()+ ";" );            }      } } 运行结果 我是公共属性 public java.lang.String com.hlmtest.java.reflect.TestFieldClass.publicStr-->我是公共属性; public static final java.lang.String com.hlmtest.java.reflect.TestFieldClass.finalStr-->110; 要注意的是，这个方法只能获取公共字段的值，私有字段的值是无法获取 的，硬来要获取则会报：java.lang.IllegalAccessException 并且会告诉你 你这个Class  can not access a member of 某类 with modifiers "private"。道理很简单，私有的属性本来就是供人家自己用的，你不可能在外面直接操作人家的。

设置字段值

可以获取字段值自然就可以设置字段值，方法是：void    set(Object obj, Object value) package com.hlmtest.java.reflect; import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; public class TestFieldObject {       public static void main(String[] args ) throws NoSuchFieldException, SecurityException, NoSuchMethodException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {             getFieldInfo ();      }             public static void getFieldInfo() throws NoSuchFieldException, SecurityException, NoSuchMethodException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException{             //先获取Class 对象实例            Class<TestFieldClass> clzz = TestFieldClass. class ;                         //获取有参的构造方法对象             @SuppressWarnings ( "rawtypes" )            Class[] paramstr = {String. class ,String. class ,String. class };            Constructor<TestFieldClass> instance = clzz .getConstructor( paramstr );             //调用Constructor 的newInstance(Object... initargs ) 方法获得主对象实例            TestFieldClass obj = instance .newInstance( "我是私有属性" , "我是公共属性" , "我是默认属性" );                         //与取值一样，私有的字段也是不可以直接设值的，所以此处只取公共字段                Field[] fieldList =  clzz .getFields();             for (Field f : fieldList ){                  //final 修饰的字段是不可设值的，否则会报错，这里得排除                  if ( f .toString().indexOf( "final" )>-1){                       continue ;                 }                  f .set( obj , f .getName()+ "我给你设值了哈" );                 Object value = f .get( obj );                 System. out .println( f .toString()+ "-->" + value .toString()+ ";" );            }      } }     运行结果： public java.lang.String com.hlmtest.java.reflect.TestFieldClass.publicStr-->publicStr我给你设值了哈;

    到此也就差不多了，大家想要了解更多的使用方法的话当然还是文档啦。

Field 的更多API可以查询：      中文网址：http://tool.oschina.net/apidocs/apidoc?api=jdk-zh      英文网址：http://tool.oschina.net/apidocs/apidoc?api=jdk_7u4

其他

其他反射相关博客：

    JDK源码/轮子分析 ：反射包 java.lang.reflect② 之 Constructor 获取类的实例对象；

    JDK源码/轮子分析 ：反射包 java.lang.reflect ①之 Class 探秘；

    201800602