Question

我在使用java反射调用方法时遇到异常，

假设我有以下课程，

package hello;

public class SimpleService {

    public void SayHello(){
        System.out.println("Hello World!");
    }
}

并将此类压缩为名为hello.jar的jar文件

然后我从另一个名为TestHello.jar的jar中的另一个类调用SayHello（），这是一个简单的Swing应用程序。

public class TestPanel extends javax.swing.JPanel{

.
.

jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
});

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        try {
            String jarPath = "/SayHello/SayHello" + ".jar";            
            URL[] classpathURLs = new URL[]{new File(jarPath).toURI().toURL()};
            ClassLoader loader = new URLClassLoader(classpathURLs);
            Class helloClass = loader.loadClass("hello.SimpleService");           
            Method sayHello = helloClass.getMethod("SayHello");            
            int modifiers = sayHello.getModifiers();

            if (Modifier.isPublic(modifiers)) {
               sayHello.invoke(this, null);
            } else {
                throw new NoSuchMethodException();
            }

        } catch (Exception ex) {
            ex.printStackTrace();
       }        
}


}
.
.

上面的TestPanel类也是使用java反射创建的，然后将该面板添加到其中一个JFrame。

所有的东西都正常工作，除非我点击jButton1我得到以下异常，

java.lang.IllegalArgumentException: object is not an instance of declaring class
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    .
    .
    .
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6504)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at java.awt.Component.processEvent(Component.java:6269)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4860)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4686)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2713)
    at java.awt.Component.dispatchEvent(Component.java:4686)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
    at java.awt.EventQueue.access$000(EventQueue.java:101)
    at java.awt.EventQueue$3.run(EventQueue.java:666)
    at java.awt.EventQueue$3.run(EventQueue.java:664)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:680)
    at java.awt.EventQueue$4.run(EventQueue.java:678)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

Answer 1

sayHello.invoke(this, null);

第一个参数应该是SimpleService。将其更改为：

sayHello.invoke(helloClass.newInstance(), null);

为清楚起见，您可以更改代码：

sayHello.invoke(helloClass.newInstance(), (Object[]) null);

或者这个：

sayHello.invoke(helloClass.newInstance());

否则可能会被误解为：

sayHello.invoke(helloClass.newInstance(), (Object) null);

或者这个：

sayHello.invoke(helloClass.newInstance(), new Object[]{null});

Answer 2

SayHello方法不带参数，因此在反射性地调用它时，除了实例之外不要传递任何内容。此外，第一个参数应该是SimpleService的实例：

替换它：

sayHello.invoke(this, null);

用这个：

sayHello.invoke(helloClass.newInstance());

Answer 3

当然，“this”的类型为TestPanel，而不是SimpleService类型

尝试以下方法：

   Class helloClass = loader.loadClass("hello.SimpleService");           

   Object args1[] = new Object[] {};
   Constructor<SimpleService> constructor = helloClass.getConstructor(args1);
   Object simpleService = constructor.newInstance();       

   sayHello.invoke(simpleService, null);

或更短

   Class helloClass = loader.loadClass("hello.SimpleService");                 
   sayHello.invoke(helloClass.newInstance(), null);

Answer 4

请注意以下一行：sayHello.invoke(this, null);

使用Method.invoke时，您需要提供Class的实例Method的实例。因此，要调用sayHello.invoke，您需要提供SimpleService的实例。

其次，由于SayHello不接受任何参数，因此调用应为sayHello.invoke(myinstance)。注意没有第二个参数。

Answer 5

有关

sayHello.invoke(this, null);

this（TestPanel）不是SimpleService的实例。您需要SimpleService的实例，或者您可以将SayHello方法设为静态：

public static void SayHello()

然后可能会传递null而不是正确性：（"If the underlying method is static, then the specified obj argument is ignored. It may be null."）

sayHello.invoke(null, null);

使用反射在java中调用方法

5 个答案: