使用Java反射时出现NoSuchMethodException

Question

我不知道出了什么问题，但我正在使用反射来调用类内部的方法。

以下是我正在做的：

ClassLoader和MAIN_PATH已经定义。我只是把它剪掉了。ClassLoader是一个URLClassLoader，它有一个指向JAR文件的URL。MAIN_PATH是org.example.BrahClass

Class mainClass = classLoader.loadClass(MAIN_PATH);
Class[] params = new Class[] { MyClass.class };
Method method = mainClass.getDeclaredMethod("onEnable", params);
MyClass myClass = new MyClass();
method.invoke(null, myClass);

该方法在以下类中声明：

public class BlahClass implements SomeInterface {
     public void onEnable(MyClass myClass){}
     public void onDisable(){}
}

完整堆栈：

    [17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]: java.lang.NoSuchMethodException: org.example.Main.commenced(org.rocket.plugin.events.PluginInitializationEvent)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at java.lang.Class.getDeclaredMethod(Class.java:2130)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at org.rocket.plugin.java.RocketPluginLoader.func_01321(RocketPluginLoader.java:42)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at org.rocket.RocketMod.setup(RocketMod.java:41)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at java.lang.reflect.Method.invoke(Method.java:497)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:537)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at java.lang.reflect.Method.invoke(Method.java:497)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.EventBus.post(EventBus.java:275)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:212)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:190)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at java.lang.reflect.Method.invoke(Method.java:497)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at com.google.common.eventbus.EventBus.post(EventBus.java:275)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:119)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at net.minecraftforge.fml.common.Loader.serverStarting(Loader.java:781)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at net.minecraftforge.fml.common.FMLCommonHandler.handleServerStarting(FMLCommonHandler.java:319)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at net.minecraft.server.dedicated.DedicatedServer.startServer(DedicatedServer.java:289)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:500)
[17:58:11] [Server thread/INFO] [STDERR]: [java.lang.Throwable$WrappedPrintStream:println:748]:     at java.lang.Thread.run(Thread.java:745)

在此处查看真实来源：https://gitlab.com/PizzaCrust/Rocket/blob/master/Rocket/src/org/rocket/plugin/java/RocketPluginLoader.java

Answer 1

NoSuchMethodException只能由mainClass.getDeclaredMethod("onEnable", params)抛出。这意味着类mainClass不具有名为onEnable的方法，该方法采用MyClass类型的单个参数。

唯一可能导致此问题的行是classLoader.loadClass(MAIN_PATH)，我敢打赌，您没有加载您认为正在加载的类，即您定义了MAIN_PATH = "org.example.BlahClass"，但您的类BlahClass不在包org.example中。另一个原因可能与classLoader有关——您也没有展示如何初始化它。

尝试添加以下检查：

if (mainClass == org.example.BlahClass.class) {
    System.out.println("It's ok!");
} else {
    System.out.println("It's not ok!");
}

另一个原因可能与CCD_ 11有关。getDeclaredMethod具有第二参数，该第二参数是var arg-Class<?>... parameterTypes。您传递了一个类型为Class[]的参数，所以它应该是好的，因为var arg只是一个数组。然而，根据JDK的不同，它可能期望第一个参数实际上是一个数组，并且您正试图获得一个具有以下签名的方法：onEnable(java.lang.Class[])（它不存在）。试着这样做：

mainClass.getDeclaredMethod("onEnable", MyClass.class)