我试图了解如何使用 URLClassLoader 访问/使 jar 文件可用。
首先,我正在加载 jar 文件
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();
}
}
打印输出显示了 jgroups-3.4.2.Final.jar 中声明的字段,但随后会引发 classnotfound 错误。
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
我不明白为什么打印输出显示该类已加载但找不到?
谢谢艺术