我正在尝试将类动态加载到组件中.我正在使用文件选择器来选择要加载的.JAR文件,然后选择一个选项窗格来获取类的名称.
我拖网了互联网,寻找如何将一个java文件转换为一个URL,以便将其加载到URLClassLoader中,我已经提出：
File myFile = filechooser.getSelectedFile();
String className = JOptionPane.showInputDialog(
this,"Class Name:","Class Name",JOptionPane.QUESTION_MESSAGE);
URL myUrl= null;
try {
myUrl = myFile.toURL();
} catch (MalformedURLException e) {
}
URLClassLoader loader = new URLClassLoader(myUrl);
loader.loadClass(className);
我现在收到一个“找不到符号”的错误,将URL加载到URLClassLoader中
解决方法
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the answer by Zellus中提到的ClassPathHacker类,但是它已经不再推荐使用了调用和坏习惯,所以这里是一个重写版本,它也缓存了Classloader和addUrl方法：
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.io.IOException;
import java.io.File;
public class ClassPathHacker{
private static final Class<URLClassLoader> URLCLASSLOADER =
URLClassLoader.class;
private static final Class<?>[] PARAMS = new Class[] { URL.class };
public static void addFile(final String s) throws IOException{
addFile(new File(s));
}
public static void addFile(final File f) throws IOException{
addURL(f.toURI().toURL());
}
public static void addURL(final URL u) throws IOException{
final URLClassLoader urlClassLoader = getUrlClassLoader();
try{
final Method method = getAddUrlMethod();
method.setAccessible(true);
method.invoke(urlClassLoader,new Object[] { u });
} catch(final Exception e){
throw new IOException(
"Error,could not add URL to system classloader");
}
}
private static Method getAddUrlMethod()
throws NoSuchMethodException{
if(addUrlMethod == null){
addUrlMethod =
URLCLASSLOADER.getDeclaredMethod("addURL",PARAMS);
}
return addUrlMethod;
}
private static URLClassLoader urlClassLoader;
private static Method addUrlMethod;
private static URLClassLoader getUrlClassLoader(){
if(urlClassLoader == null){
final ClassLoader sysloader =
ClassLoader.getSystemClassLoader();
if(sysloader instanceof URLClassLoader){
urlClassLoader = (URLClassLoader) sysloader;
} else{
throw new IllegalStateException(
"Not an UrlClassLoader: "
+ sysloader);
}
}
return urlClassLoader;
}
}
