为什么Java找不到我的班级？

Question

I have a class that compiles without error. The class has a main method. But when I try to run it on ubuntu linux from the classes' directory I get a class not found error. I am pretty sure I am missing something dead obvious but I don't see it.

Here is my ls operation:

zookeeper@zookeeper-virtual-machine:~/zookeeper-3.4.5/programs$ ls
CreateGroup.java  LsGroup.class  LsGroup.java  zookeeper-3.4.5.jar

Here is what happens when I to run LsGroup

zookeeper@zookeeper-virtual-machine:~/zookeeper-3.4.5/programs$ java -cp "zookeeper-3.4.5.jar" LsGroup
Exception in thread "main" java.lang.NoClassDefFoundError: LsGroup
Caused by: java.lang.ClassNotFoundException: LsGroup
    at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: LsGroup. Program will exit.

Here is the code for LsGroup

package org.zookeeper;    
import java.io.IOException;
import java.util.List;
import java.util.concurrent.CountDownLatch;

import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.Watcher.Event.KeeperState;

public class LsGroup implements Watcher {

    private static final int SESSION_TIMEOUT = 5000;
    private ZooKeeper zk;
    private CountDownLatch connectedSignal = new CountDownLatch(1);

    public void connect(String hosts) throws IOException, InterruptedException {
        zk = new ZooKeeper(hosts, SESSION_TIMEOUT, this);
        connectedSignal.await();
    }

    @Override
    public void process(WatchedEvent event) { // Watcher interface
        if (event.getState() == KeeperState.SyncConnected) {
            connectedSignal.countDown();
        }
    }

    public void ls(String groupName) throws KeeperException, InterruptedException {
        String path = "/" + groupName;
        try {
            List<String> children = zk.getChildren(path, false);
            for (String child : children) {
                System.out.println(path+"/"+child);
                System.out.println(zk.getChildren(path +"/"+ child, false));
            }
        } catch (KeeperException.NoNodeException e) {
            System.out.printf("Group %s does not exist\n", groupName);
            System.exit(1);
        }
    }

    public void close() throws InterruptedException {
        zk.close();
    }

    public static void main(String[] args) throws Exception {
        LsGroup lsGroup = new LsGroup();
        lsGroup.connect(args[0]);
        lsGroup.ls(args[1]);
        lsGroup.close();
    }
}

Answer 1

The original problem was that your class was in a package, but you were trying to load it as if it weren't in a package. You'd normally organize your source code to match your package hierarchy, then from the root of the hierarchy, you'd run something like:

java -cp .:zookeeper-3.4.5.jar org.zookeeper.LsGroup

Now that you've temporarily worked around the package issue by moving the code out of a package, the next problem is that the current directory isn't in the classpath. So instead of this:

java -cp "zookeeper-3.4.5.jar" LsGroup

You want:

java -cp .:zookeeper-3.4.5.jar LsGroup

Once you've got that working, you should move the classes back into packages, as per normal Java best practice.

Answer 2

You could remove the package declaration:

package org.zookeeper;

...or just place your LsGroup class in org/zookeeper directory.

Answer 3

The class file is supposed to live in a path like:

 org/zookeeper/LsGroup.class

The -cp must include the directory that contains the org/ directory. Then you can

java -cp parent-of-org org.zookeeper.LsGroup

Answer 4

The message "wrong name: org/zookeeper/LsGroup" means, you have to respect the package structure of Java. Use the following directory structure:

./org/zookeeper/LsGroup.class

Then launch java org.zookeeper.LsGroup from within the current directory. The package separator "." will be translated to corresponding directory.

Answer 5

Your files are not under package org.zookeeper

You should be running your class from ~/zookeeper-3.4.5/org/zookeeper

Otherwise JVM won't find the classes to load.

Answer 6

Your class is part of the org.zookeeper package but you keep it in the root folder of your project ( /zookeeper-3.4.5/programs ).
The qualified name of the package member and the path name to the file are parallel ( see Managing Source and Class Files ), so if your package is org.zookeeper the class file should be kept in /zookeeper-3.4.5/programs/org/zookeeper

Answer 7

You made two mistakes:

1) You tried to run java LsGroup but you have to use the complete name including packages java org.zookeeper.LsGroup

2) your directory structure is not correct: the package org.zookeeper corresponds with the directory structure ./org/zookeeper/

If you change the directory structure and then run java from the top of this directory structure it should work