在開始之前,我先定義一個測試類Student,代碼如下:
package chb.test.reflect;
public class Student {
private int age;
private String name;
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public static void hi(int age,String name){
System.out.println("大家好,我叫"+name+",今年"+age+"歲");
}
}
一、JAVA反射的常規使用步驟
反射調用一般分爲3個步驟:
得到要調用類的class
得到要調用的類中的方法(Method)
方法調用(invoke)
代碼示例:
Class cls = Class.forName("chb.test.reflect.Student");
Method m = cls.getDeclaredMethod("hi",new Class[]{int.class,String.class});
m.invoke(cls.newInstance(),20,"chb");
二、方法調用中的參數類型
在方法調用中,參數類型必須正確,這裏需要注意的是不能使用包裝類替換基本類型,比如不能使用Integer.class代替int.class。
如我要調用Student的setAge方法,下面的調用是正確的:
Class cls = Class.forName("chb.test.reflect.Student");
Method setMethod = cls.getDeclaredMethod("setAge",int.class);
setMethod.invoke(cls.newInstance(), 15);
而如果我們用Integer.class替代int.class就會出錯,如:
Class cls = Class.forName("chb.test.reflect.Student");
Method setMethod = cls.getDeclaredMethod("setAge",Integer.class);
setMethod.invoke(cls.newInstance(), 15);
jvm會報出如下異常:
java.lang.NoSuchMethodException: chb.test.reflect.Student.setAge(java.lang.Integer)
at java.lang.Class.getDeclaredMethod(Unknown Source)
at chb.test.reflect.TestClass.testReflect(TestClass.java:23)
三、static方法的反射調用
static方法調用時,不必得到對象示例,如下:
Class cls = Class.forName("chb.test.reflect.Student");
Method staticMethod = cls.getDeclaredMethod("hi",int.class,String.class);
staticMethod.invoke(cls,20,"chb");//這裏不需要newInstance
//staticMethod.invoke(cls.newInstance(),20,"chb");四、private的成員變量賦值
如果直接通過反射給類的private成員變量賦值,是不允許的,這時我們可以通過setAccessible方法解決。代碼示例:
Class cls = Class.forName("chb.test.reflect.Student");
Object student = cls.newInstance();//得到一個實例
Field field = cls.getDeclaredField("age");
field.set(student, 10);
System.out.println(field.get(student));
運行如上代碼,系統會報出如下異常:
java.lang.IllegalAccessException: Class chb.test.reflect.TestClass can not access a member of class chb.test.reflect.Student with modifiers "private"
at sun.reflect.Reflection.ensureMemberAccess(Unknown Source)
at java.lang.reflect.Field.doSecurityCheck(Unknown Source)
at java.lang.reflect.Field.getFieldAccessor(Unknown Source)
at java.lang.reflect.Field.set(Unknown Source)
at chb.test.reflect.TestClass.testReflect(TestClass.java:20)
解決方法:
Class cls = Class.forName("chb.test.reflect.Student");
Object student = cls.newInstance();
Field field = cls.getDeclaredField("age");
field.setAccessible(true);//設置允許訪問
field.set(student, 10);
System.out.println(field.get(student)); 其實,在某些場合下(類中有get,set方法),可以先反射調用set方法,再反射調用get方法達到如上效果,代碼示例:
Class cls = Class.forName("chb.test.reflect.Student");
Object student = cls.newInstance();
Method setMethod = cls.getDeclaredMethod("setAge",Integer.class);
setMethod.invoke(student, 15);//調用set方法
Method getMethod = cls.getDeclaredMethod("getAge");
System.out.println(getMethod.invoke(student));//再調用get方法