java使用反射来调用公共方法
我正在编写一个API,在使用反射时遇到了一个问题。我要做的是从自定义对象类调用一个方法,并将值返回到调用的位置。调用的开始发生在main活动中。爪哇
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ByteMe b = new ByteMe();
ExampleObject object = new ExampleObject("Bob", 20, "indy", "male", "its bobby");
try {
b.examine(object);
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
}
我的示例对象类旨在帮助确保该API正常工作
public class ExampleObject {
private String Name;
private int Age;
private String Location;
private String Sex;
private String Description;
/**
* Empty Constructor
*/
public ExampleObject() {}
/**
* Basic constructor with initializing data
*
* @param _Name String with the name of the user
* @param _Age Integer with the age of the user
* @param _Location String containing the curret city and state of the user
* @param _Sex String Male, Female, Transgender, or Other
* @param _Description String short blurb about the user
*/
public ExampleObject(String _Name, int _Age, String _Location, String _Sex, String _Description)
{
this.setName(_Name);
this.setAge(_Age);
this.setLocation(_Location);
this.setSex(_Sex);
this.setDescription(_Description);
}
public String getName() {
return Name;
}
public void setName(String name) {
Name = name;
}
public int getAge() {
return Age;
}
public void setAge(int age) {
Age = age;
}
public String getLocation() {
return Location;
}
public void setLocation(String location) {
Location = location;
}
public String getSex() {
return Sex;
}
public void setSex(String sex) {
Sex = sex;
}
public String getDescription() {
return Description;
}
public void setDescription(String description) {
Description = description;
}
}
现在我面临的主要问题是,当我在另一个名为ByteMe的文件中调用时。爪哇:
public void examine(Object obj) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
//Get the list of possible methods held in this object.
Method[] methods = obj.getClass().getMethods();
// iterate through them
for (Method method : methods) {
Log.d("" + this.getClass().getName(), "--------------------------");
Log.d("" + this.getClass().getName(), "Method: " + method.getName());
Log.d("" + this.getClass().getName(), "Return Type: " + method.getReturnType());
Log.d("" + this.getClass().getName(), "Class: " + method.getClass());
Log.d("" + this.getClass().getName(), "Declaring Class: " + method.getDeclaringClass());
if(method.getReturnType().getName().contains("int")) {
try {
Method m = method.getDeclaringClass().getMethod(method.getName(), Integer.TYPE);
int temp = (int) m.invoke(null, 0); //first argument is the object to invoke on, ignored if static method
Log.d("" + this.getClass().getName(),"temp value: " + temp);
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
Log.d("" + this.getClass().getName(), "--------------------------");
}
}
这是我的日志摘要:
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: --------------------------
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: Method: equals
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: Return Type: boolean
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: Class: class java.lang.reflect.Method
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: Declaring Class: class java.lang.Object
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: --------------------------
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: --------------------------
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: Method: getAge
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: Return Type: int
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: Class: class java.lang.reflect.Method
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: Declaring Class: class productions.widowmaker110.byteme.ExampleObject
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: java.lang.NoSuchMethodException: getAge [int]
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at java.lang.Class.getMethod(Class.java:664)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at java.lang.Class.getMethod(Class.java:643)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at Library.ByteMe.examine(ByteMe.java:94)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at productions.widowmaker110.byteme.MainActivity.onCreate(MainActivity.java:21)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at 安卓.app.Activity.performCreate(Activity.java:5958)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at 安卓.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1129)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2364)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at 安卓.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2474)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at 安卓.app.ActivityThread.access$800(ActivityThread.java:144)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1359)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at 安卓.os.Handler.dispatchMessage(Handler.java:102)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at 安卓.os.Looper.loop(Looper.java:155)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at 安卓.app.ActivityThread.main(ActivityThread.java:5696)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at java.lang.reflect.Method.invoke(Native Method)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at java.lang.reflect.Method.invoke(Method.java:372)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme W/System.err: at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:823)
11-21 20:08:11.704 18121-18121/productions.widowmaker110.byteme D/Library.ByteMe: --------------------------
我查找了NoSuchMethodFound错误,大多数stackoverflow说这是由于方法是private
,但我的方法是public
。examine()中的invoke方法应该返回20,因为这是我在main活动中创建的对象的年龄。但它只是抛出了一个例外。帮忙
编辑Alain O'Dea,谢谢你的帮助。这是我必须改变的,才能让它发挥作用
public void examine(Object obj) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
//Get the list of possible methods held in this object.
Method[] methods = obj.getClass().getMethods();
// iterate through them
for (Method method : methods) {
Log.d("" + this.getClass().getName(), "--------------------------");
Log.d("" + this.getClass().getName(), "Method: " + method.getName());
Log.d("" + this.getClass().getName(), "Return Type: " + method.getReturnType());
Log.d("" + this.getClass().getName(), "Class: " + method.getClass());
Log.d("" + this.getClass().getName(), "Declaring Class: " + method.getDeclaringClass());
if(method.getReturnType().getName().contains("int")) {
try {
int temp = (int) method.invoke(obj);
Log.d("" + this.getClass().getName(),"temp value: " + temp);
} catch (InvocationTargetException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
Log.d("" + this.getClass().getName(), "--------------------------");
}
}
logcat
11-21 20:48:15.459 26949-26949/productions.widowmaker110.byteme D/Library.ByteMe: --------------------------
11-21 20:48:15.469 26949-26949/productions.widowmaker110.byteme D/Library.ByteMe: Method: getAge
11-21 20:48:15.469 26949-26949/productions.widowmaker110.byteme D/Library.ByteMe: Return Type: int
11-21 20:48:15.469 26949-26949/productions.widowmaker110.byteme D/Library.ByteMe: Class: class java.lang.reflect.Method
11-21 20:48:15.469 26949-26949/productions.widowmaker110.byteme D/Library.ByteMe: Declaring Class: class productions.widowmaker110.byteme.ExampleObject
11-21 20:48:15.469 26949-26949/productions.widowmaker110.byteme D/Library.ByteMe: temp value: 20
11-21 20:48:15.469 26949-26949/productions.widowmaker110.byteme D/Library.ByteMe: --------------------------
# 1 楼答案
如果查看类的docs,您将看到
getMethod(String name, Class...<?> parameterTypes)
接受表示方法名(“getAge”在本例中)和参数类型(Integer)的参数。您正在访问的类没有具有该签名(public int getAge(int age)
)的方法,因此出现了NoSuchMethodException。这不是在骗你另外,既然您已经有了要逐步执行的方法,为什么不直接调用
m.invoke(obj)
(如果它是正确的方法),而不是再次查询类中的方法呢?这可能会把事情弄清楚一点最后,正如你的评论所说,当你调用
invoke()
时,你需要传递你正在调用方法的对象,除非它是静态的(事实并非如此)。所以你需要把它改成m.invoke(obj)
# 2 楼答案
您有以下代码:
结果得到了这个堆栈跟踪:
值得注意的是,getAge(int)是一种非静态(实例)方法:
触发该异常的问题源于以下代码:
这需要一个不存在的方法int methodName(例如int getAge(int))来触发NoSuchMethodException。这是一个多余的步骤,因为您可以只调用方法上的调用
不过,这并不能单独起作用,因为调用也有问题。因此,必须对对象进行反射调用,而不是null。这也不是全部问题。方法是int getAge()而不是int getAge(int)。它不需要int参数。因此,您还需要从调用中删除0参数
下面是正确的一行:
对象方法的文档。调用(对象,对象…)可供参考:
资料来源:http://docs.oracle.com/javase/7/docs/api/java/lang/reflect/Method.html#invoke(java.lang.Object,%20java.lang.Object...)