Java RMI和ClassNotFoundException

我刚刚开始学习如何使用RMI，我有一个问题。我有以下目录结构：

compute.jar
client
     |
     org\examples\rmi\client
                           |--> ComputePi     // client main
                           |--> Pi            // implements Task
     org\examples\rmi\compute
                           |--> Compute       // interface
                           |--> Task          // interface

server
     |
     org\examples\rmi\engine
                           |--> ComputeEngine // server main, implements Compute
     org\examples\rmi\compute
                           |--> Compute       // interface
                           |--> Task          // interface

下面是

ComputePi类中的
main
方法：

if (System.getSecurityManager() == null) {
  System.setSecurityManager(new SecurityManager());
}
try {
  String name = "Compute";
  // args[0] = 127.0.0.1, args[1] is irrelevant
  Registry registry = LocateRegistry.getRegistry(args[0], 0);
  Compute comp = (Compute) registry.lookup(name);
  Pi task = new Pi(Integer.parseInt(args[1]));
  BigDecimal pi = comp.executeTask(task);
  System.out.println(pi);
}
catch (Exception e) {
  System.err.println("ComputePi exception:");
  e.printStackTrace();
}

if (System.getSecurityManager() == null) {
  System.setSecurityManager(new SecurityManager());
}
try {
  String name = "Compute";
  Compute engine = new ComputeEngine();
  Compute stub = (Compute) UnicastRemoteObject.exportObject(engine, 0);
  Registry registry = LocateRegistry.getRegistry();
  registry.rebind(name, stub);
  System.out.println("ComputeEngine bound.");
}
catch (Exception e) {
  System.err.println("ComputeEngine exception: ");
  e.printStackTrace();
}

  public <T> T executeTask(Task<T> task) throws RemoteException {
    if (task == null) {
      throw new IllegalArgumentException("task is null");
    }
    return task.execute();
  }

以下是
ComputeEngine类中的
main
方法：

if (System.getSecurityManager() == null) {
  System.setSecurityManager(new SecurityManager());
}
try {
  String name = "Compute";
  // args[0] = 127.0.0.1, args[1] is irrelevant
  Registry registry = LocateRegistry.getRegistry(args[0], 0);
  Compute comp = (Compute) registry.lookup(name);
  Pi task = new Pi(Integer.parseInt(args[1]));
  BigDecimal pi = comp.executeTask(task);
  System.out.println(pi);
}
catch (Exception e) {
  System.err.println("ComputePi exception:");
  e.printStackTrace();
}

if (System.getSecurityManager() == null) {
  System.setSecurityManager(new SecurityManager());
}
try {
  String name = "Compute";
  Compute engine = new ComputeEngine();
  Compute stub = (Compute) UnicastRemoteObject.exportObject(engine, 0);
  Registry registry = LocateRegistry.getRegistry();
  registry.rebind(name, stub);
  System.out.println("ComputeEngine bound.");
}
catch (Exception e) {
  System.err.println("ComputeEngine exception: ");
  e.printStackTrace();
}

  public <T> T executeTask(Task<T> task) throws RemoteException {
    if (task == null) {
      throw new IllegalArgumentException("task is null");
    }
    return task.execute();
  }

以下是
executeTask
方法，也在ComputeEngine类中：

if (System.getSecurityManager() == null) {
  System.setSecurityManager(new SecurityManager());
}
try {
  String name = "Compute";
  // args[0] = 127.0.0.1, args[1] is irrelevant
  Registry registry = LocateRegistry.getRegistry(args[0], 0);
  Compute comp = (Compute) registry.lookup(name);
  Pi task = new Pi(Integer.parseInt(args[1]));
  BigDecimal pi = comp.executeTask(task);
  System.out.println(pi);
}
catch (Exception e) {
  System.err.println("ComputePi exception:");
  e.printStackTrace();
}

if (System.getSecurityManager() == null) {
  System.setSecurityManager(new SecurityManager());
}
try {
  String name = "Compute";
  Compute engine = new ComputeEngine();
  Compute stub = (Compute) UnicastRemoteObject.exportObject(engine, 0);
  Registry registry = LocateRegistry.getRegistry();
  registry.rebind(name, stub);
  System.out.println("ComputeEngine bound.");
}
catch (Exception e) {
  System.err.println("ComputeEngine exception: ");
  e.printStackTrace();
}

  public <T> T executeTask(Task<T> task) throws RemoteException {
    if (task == null) {
      throw new IllegalArgumentException("task is null");
    }
    return task.execute();
  }

以下是客户端的参数：

C:\Users\Public\RMI\client>java -Djava.rmi.server.codebase="file:/C:/Users/Public/RMI/compute.jar" -Djava.security.policy=client.policy org.examples.rmi.client.ComputePi 127.0.0.1 45

java.rmi.ServerException: RemoteException occurred in server thread; nested exception is:
        java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.lang.ClassNotFoundException: org.examples.rmi.client.Pi
        at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
        at sun.rmi.transport.StreamRemoteCall.exceptionReceivedFromServer(Unknown Source)
        at sun.rmi.transport.StreamRemoteCall.executeCall(Unknown Source)
        at sun.rmi.server.UnicastRef.invoke(Unknown Source)
        at java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(Unknown Source)
        at java.rmi.server.RemoteObjectInvocationHandler.invoke(Unknown Source)
        at $Proxy0.executeTask(Unknown Source)
        at org.examples.rmi.client.ComputePi.main(ComputePi.java:38)
Caused by: java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.lang.ClassNotFoundException: org.examples.rmi.client.Pi
        at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: org.examples.rmi.client.Pi
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Unknown Source)
        at sun.rmi.server.LoaderHandler.loadClass(Unknown Source)
        at sun.rmi.server.LoaderHandler.loadClass(Unknown Source)
        at java.rmi.server.RMIClassLoader$2.loadClass(Unknown Source)
        at java.rmi.server.RMIClassLoader.loadClass(Unknown Source)
        at sun.rmi.server.MarshalInputStream.resolveClass(Unknown Source)
        at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
        at java.io.ObjectInputStream.readClassDesc(Unknown Source)
        at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
        at java.io.ObjectInputStream.readObject0(Unknown Source)
        at java.io.ObjectInputStream.readObject(Unknown Source)
        at sun.rmi.server.UnicastRef.unmarshalValue(Unknown Source)
        ... 11 more

但是，当我尝试运行客户端时，会出现以下异常：

C:\Users\Public\RMI\client>java -Djava.rmi.server.codebase="file:/C:/Users/Public/RMI/compute.jar" -Djava.security.policy=client.policy org.examples.rmi.client.ComputePi 127.0.0.1 45

java.rmi.ServerException: RemoteException occurred in server thread; nested exception is:
        java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.lang.ClassNotFoundException: org.examples.rmi.client.Pi
        at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
        at sun.rmi.transport.StreamRemoteCall.exceptionReceivedFromServer(Unknown Source)
        at sun.rmi.transport.StreamRemoteCall.executeCall(Unknown Source)
        at sun.rmi.server.UnicastRef.invoke(Unknown Source)
        at java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(Unknown Source)
        at java.rmi.server.RemoteObjectInvocationHandler.invoke(Unknown Source)
        at $Proxy0.executeTask(Unknown Source)
        at org.examples.rmi.client.ComputePi.main(ComputePi.java:38)
Caused by: java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
        java.lang.ClassNotFoundException: org.examples.rmi.client.Pi
        at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at sun.rmi.transport.Transport$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: org.examples.rmi.client.Pi
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Unknown Source)
        at sun.rmi.server.LoaderHandler.loadClass(Unknown Source)
        at sun.rmi.server.LoaderHandler.loadClass(Unknown Source)
        at java.rmi.server.RMIClassLoader$2.loadClass(Unknown Source)
        at java.rmi.server.RMIClassLoader.loadClass(Unknown Source)
        at sun.rmi.server.MarshalInputStream.resolveClass(Unknown Source)
        at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
        at java.io.ObjectInputStream.readClassDesc(Unknown Source)
        at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
        at java.io.ObjectInputStream.readObject0(Unknown Source)
        at java.io.ObjectInputStream.readObject(Unknown Source)
        at sun.rmi.server.UnicastRef.unmarshalValue(Unknown Source)
        ... 11 more

但是如果我将Pi.class文件添加到服务器目录：

server
     |
     org\examples\rmi\engine
                           |--> ComputeEngine // server main, implements Compute
     org\examples\rmi\compute
                           |--> Compute       // interface
                           |--> Task          // interface
     org\examples\rmi\client
                           |--> Pi            // same as Pi for client

这个程序有效。我的问题是，Pi.class真的需要在服务器上运行我的程序吗？我的理解是（如果我错了，请纠正我），我将该类的一个实例发送到服务器，服务器将知道如何处理它，也就是说，它不关心实现。有人能解释一下RMI在我的情况下是如何工作的吗？我真的很感激。谢谢
 您正试图发送服务器未知的类的序列化对象

执行时：

  Pi task = new Pi(Integer.parseInt(args[1]));
  BigDecimal pi = comp.executeTask(task);

服务器并不真正知道什么是
Pi
。由于
Pi
类是API的一部分，所以它也应该加载到服务器上

当我有一个应用程序需要远程执行某些操作时，例如使用
RMI
、Spring Remoting或类似工具，我将我的项目分为3个项目：
API
、服务器和客户端。API项目将具有与功能相关的所有接口和模型类（该项目将生成一个jar，或多或少类似于您的计算机jar）。服务器将导入API JAR，实现接口，并通过远程层（就像您对服务器所做的那样）使服务可用，而客户机将与您对客户机所做的一样

在处理序列化时，双方都必须知道类本身。然后传输对象的状态，以便在另一侧重建对象

序列化是RMI用来在对象之间传递对象的机制
JVM，作为从客户端到客户端的方法调用中的参数
服务器或作为方法调用的返回值

威廉·格罗索（2001年10月）。还有更多信息。
我认为您为客户端指定的代码库不正确：

-Djava.rmi.server.codebase=“文件：/C:/Users/Public/rmi/compute.jar”

这不会帮助服务器找到client.Pi

“Java教程RMI”指定

-Djava.rmi.server.codebase=文件：/c:/home/jones/public\u html/classes/

这是client/Pi.class所在的目录（也就是说，如果遵循教程，“jones”编写了client）

不幸的是，即使我听从自己的建议，也意味着在我的情况下

-Djava.rmi.server.codebase=file:/h:/rmi-example/jones/src/

当我启动客户机时，我会得到与您相同的异常

还没解决呢。我希望java option
-verbose:class
能对这个问题有所帮助。
我在同一网络中的两台PC上尝试了这个例子。一个使用Java1.7.0_40作为服务器，另一个使用Java1.7.0_45作为客户端。这两台电脑都是基于Windows的。我遇到了denshaotoko提出的同样问题

解决办法是：

服务器端：

C:\>start rmiregistry -J-Djava.rmi.server.useCodebaseOnly=false

Java 7需要-J-Djava.rmi.server.useCodebaseOnly选项，因为默认值为true，这意味着rmi注册表将不会查找除其启动目录以外的其他代码库。然后，启动服务器的下一步将失败。详情请参阅：

同样，java.rmi.server.useCodebaseOnly应设置为false。否则，服务器将不会使用客户端提供的代码库。然后客户端将获得类NotFound异常。主机名192.168.1.124是服务器的IP地址

你应该得到“计算机引擎绑定”

客户端：

C:\>java -cp c:\rmi;c:\rmi\compute.jar -Djava.rmi.server.codebase=http://54.200.126.244/rmi/ -Djava.security.policy=c:\rmi\client.policy client.ComputePi 192.168.1.124 45 

我编辑了文件：/URL，但未成功。我认为原因很简单。有太多的安全限制使得服务器无法访问客户端PC上的文件。因此，我将Pi.class文件放在我的web服务器上，
http://54.200.126.244
在rmi目录下。我的web服务器使用Apache。任何电脑都可以访问
http://54.200.126.244/rmi/
因此问题得到了彻底解决

最后，您应该能够使用相同的命令从任何目录启动rmiregistry、服务器和客户机。否则，即使成功，某些设置可能仍然不正确。例如，如果您从包含“compute”目录的目录（在我的示例中是C:\rmi）启动rmiregistry，rmiregistry将直接从其启动目录加载compute.class和Task.class，因此-Djava.rmi.server.codebase=file:/C:/rmi/compute.jar的设置将变得无用

我的问题是，Pi.class真的需要在服务器上运行我的程序吗？我的理解是（如果我错了，请纠正我），我将该类的一个实例发送到服务器，服务器将知道如何处理它，也就是说，它不关心实现

你理解对了。编译Pi.class时，它不需要在服务器上，但服务器需要在运行时下载它！（Pi必须是可序列化的）

问题是：服务器如何知道在哪里下载Pi.class，何时需要它

答案是：根据客户机提供的java.rmi.server.codebase设置值。客户端必须设置java.rmi.server.codebase选项。你必须说Pi.class在哪里。将Pi.class的副本放在公共目录中进行部署是一种常见的习惯。因此，完整的解决方案是：

结构：

compute.jar
client\
|-org\
|   |-examples\
|       |-rmi\
|           |client\
|               |--> ComputePi     // client main
|               |--> Pi            // implements Task
|-deploy\            
|   |-org\
|       |-examples\
|           |-rmi\
|               |-client\ // directory that will contain the deployment copy of Pi.class
|--> client.policy  

server\
|-org\
|   |-examples\
|       |-rmi\
|           |-engine\
|               |--> ComputeEngine // server main, implements Compute
|--> server.policy

其中compute.jar是以前创建的jar文件

cd C:\Users\Public\RMI\
javac compute\Compute.java compute\Task.java
jar cvf compute.jar compute\*.class

您是否在java文件中正确设置了package和import命令？因为你是