问题
I am trying to understand how to access/make available a jar file using URLClassLoader.
Firstly I am loading the jar file with
package myA;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import org.jgroups.JChannel;
public class loader {
JChannel channel;
String user_name=System.getProperty("user.name", "n/a");
private void start() throws Exception {
channel=new JChannel(); // use the default config, udp.xml
channel.connect("ChatCluster");
}
public void loadMe()throws ClassNotFoundException, MalformedURLException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
URL classUrl;
classUrl = new URL("file:///home/myJars/jgroups-3.4.2.Final.jar");
URL[] classUrls = { classUrl };
URLClassLoader ucl = new URLClassLoader(classUrls);
Class<?> c = ucl.loadClass("org.jgroups.JChannel");
for(Field f: c.getDeclaredFields()) {
System.out.println("Field name=" + f.getName());
}
Object instance = c.newInstance();
//Method theMethod = c.getMethod("main");
//theMethod.invoke(instance);
}
public static void main(String[] args) throws Exception {
new loader().loadMe();
new loader().start();
}
}
the printout shows the declared fields that are in jgroups-3.4.2.Final.jar, however it then throws a classnotfound error.
java -cp myA.jar myA.loader
Field name=DEFAULT_PROTOCOL_STACK
Field name=local_addr
Field name=address_generator
Field name=name
Field name=cluster_name
Field name=my_view
Field name=prot_stack
Field name=state_promise
Field name=state_transfer_supported
Field name=flush_supported
Field name=config
Field name=stats
Field name=sent_msgs
Field name=received_msgs
Field name=sent_bytes
Field name=received_bytes
Field name=probe_handler
Exception in thread "main" java.lang.NoClassDefFoundError: org/jgroups/JChannel
at myA.loader.start(loader.java:23)
at myA.loader.main(loader.java:45)
Caused by: java.lang.ClassNotFoundException: org.jgroups.JChannel
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
... 2 more
I don't understand why the printout shows that the class is loaded but then it is not found?
thx Art
回答1:
Your code has maybe a couple of problems. First, you instantiate the loader 2 times within main, so the second instance is independent from the first one and might not be aware that the first one loaded the class file definition of JChannel
.
Moreover you've defined JChannel as a member of loader
before, therefore the JRE should require the class definition for it at startup - else it should not know what this field should be. I've replaced it with the class you've loaded via the URLClassLoader
which you should instantiate in start()
.
package myA;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import org.jgroups.JChannel;
public class Loader
{
Class<?> clazz;
String user_name=System.getProperty("user.name", "n/a");
private void start() throws Exception
{
if (this.clazz == null)
throw new Exception("Channel class was not loaded properly");
Object channel = this.clazz.newInstance(); // use the default config, udp.xml
Method chatCluster = this.clazz.getDeclaredMethod("connect", new Class[] { String.class });
chatCluster.invoke(channel, "ChatCluster");
}
public void loadMe() throws Exception
{
URL classUrl;
classUrl = new URL("file:///home/myJars/jgroups-3.4.2.Final.jar");
URL[] classUrls = { classUrl };
URLClassLoader ucl = new URLClassLoader(classUrls);
Class<?> c = ucl.loadClass("org.jgroups.JChannel");
for(Field f: c.getDeclaredFields())
{
System.out.println("Field name=" + f.getName());
}
this.clazz = c;
Object instance = c.newInstance();
//Method theMethod = c.getMethod("main");
//theMethod.invoke(instance);
}
public static void main(String[] args) throws Exception
{
Loader loader = new Loader();
loader.loadMe();
loader.start();
}
}
You should further add some error handling to the code.
回答2:
Exception in thread "main" java.lang.NoClassDefFoundError: org/jgroups/JChannel
at myA.loader.start(loader.java:23)
The code fails on this line:
channel=new JChannel(); // use the default config, udp.xml
The type JChannel is not visible to loader's ClassLoader. This will be obvious if you try:
loader.class
.getClassLoader()
.loadClass("org.jgroups.JChannel");
You should not have any compile-time references to a dependency that will not be on the type's classpath at runtime.
Loading with a new child ClassLoader does not add that type to some global class pool. The loaders are hierarchical with child-parent relationships.
来源:https://stackoverflow.com/questions/21073986/understanding-urlclassloader-how-to-access-a-loaded-jars-classes