Parent Last Classloader解决Java类路径地狱？ [英] Parent Last Classloader to solve Java Class path hell?
问题描述
我有一个项目，它使用两个版本的bouncyCastle jars bcprov-jdk15和bcprov-jdk16。 jvm加载旧版本，但我写的一个功能需要运行更新的版本。我试图通过使用自定义类加载器来解决这个类路径地狱。经过一些谷歌搜索并在之前的一些Stackoverflow答案的帮助下 [1 ] [2] 和博客，我编写了以下父级最后一个类加载器，以便在委托给父类加载器之前从较新的jar加载类。
I have a project which uses two versions of bouncyCastle jars bcprov-jdk15 and bcprov-jdk16. The jvm loads the older version but there is a feature I wrote which needs the newer version to run. I tried to solve this classpath hell by using a custom class loader. After some googling and with the help of some previous Stackoverflow answers[1] [2] and this blog, I wrote the following Parent Last Class loader to load the classes from the newer jar before delegating to the parent class loader.
public class ParentLastClassLoader extends ClassLoader {
private String jarFile; //Path to the jar file
private Hashtable classes = new Hashtable(); //used to cache already defined classes
public ParentLastClassLoader(ClassLoader parent, String path)
{
super(parent);
this.jarFile = path;
}
@Override
public Class<?> findClass(String name) throws ClassNotFoundException
{
System.out.println("Trying to find");
throw new ClassNotFoundException();
}
@Override
protected synchronized Class<?> loadClass(String className, boolean resolve) throws ClassNotFoundException
{
System.out.println("Trying to load");
try
{
System.out.println("Loading class in Child : " + className);
byte classByte[];
Class result = null;
//checks in cached classes
result = (Class) classes.get(className);
if (result != null) {
return result;
}
try {
JarFile jar = new JarFile(jarFile);
JarEntry entry = jar.getJarEntry(className + ".class");
InputStream is = jar.getInputStream(entry);
ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
int nextValue = is.read();
while (-1 != nextValue) {
byteStream.write(nextValue);
nextValue = is.read();
}
classByte = byteStream.toByteArray();
result = defineClass(className, classByte, 0, classByte.length, null);
classes.put(className, result);
return result;
} catch (Exception e) {
throw new ClassNotFoundException(className + "Not found", e);
}
}
catch( ClassNotFoundException e ){
System.out.println("Delegating to parent : " + className);
// didn't find it, try the parent
return super.loadClass(className, resolve);
}
}
}
我加载了主类此类加载器的功能，但功能中使用的BouncyCaslte类不由我的自定义类加载器加载。
I loaded the main class in the feature with this class loader but the BouncyCaslte classes used in the feature are not loaded by my custom classloader.
ClassLoader loader = new ParentLastClassLoader(Thread.currentThread().getContextClassLoader(), pathToJar);
Class myClass = loader.loadClass("MainClassOfTheFeature");
Method mainMethod = myClass.getMethod("MainMethod");
mainMethod.invoke(myClass.getConstructor().newInstance());
Jvm仍然使用从旧版本加载的类。如何在运行该功能时让JVM从我的类加载器加载类，并在功能未运行时使用旧jar中已加载的旧类？
Jvm still uses the classes it loaded from the older version. How can I make the JVM to load the classes from my class loader when running the feature and use the already loaded older classes in the older jar when the feature is not running?
编辑：
即使将自定义类加载器设置为要素Main类的MainMethod中的Thread上下文类加载器，问题仍然存在。
The problem remains even after setting the custom classloader as the Thread context classloader in the MainMethod of the feature Main class.
Thread.currentThread().setContextClassLoader(this.getClass().getClassLoader());
推荐答案
我设法解决了这个问题。修改了 ParentLastClassLoader 的代码，以获取该功能所需的所有Jarfile路径的数组。因此，当加载类时，将搜索该功能所需的所有jar文件以查找.class文件。如果找不到类文件，它将被委托给父类。
I managed to solve this problem. Modified the code of the ParentLastClassLoader to get an array of all the Jarfile paths which are needed by the feature. So when a class is loaded, all the jarfiles needed by the feature will be searched for the .class files. If a class file cannot be found, it will be delegated to the parent.
private class ParentLastClassLoader extends ClassLoader {
private String[] jarFiles; //Paths to the jar files
private Hashtable classes = new Hashtable(); //used to cache already defined classes
public ParentLastClassLoader(ClassLoader parent, String[] paths)
{
super(parent);
this.jarFiles = paths;
}
@Override
public Class<?> findClass(String name) throws ClassNotFoundException
{
System.out.println("Trying to find");
throw new ClassNotFoundException();
}
@Override
protected synchronized Class<?> loadClass(String className, boolean resolve) throws ClassNotFoundException
{
System.out.println("Trying to load");
try
{
System.out.println("Loading class in Child : " + className);
byte classByte[];
Class result = null;
//checks in cached classes
result = (Class) classes.get(className);
if (result != null) {
return result;
}
for(String jarFile: jarFiles){
try {
JarFile jar = new JarFile(jarFile);
JarEntry entry = jar.getJarEntry(className.replace(".","/") + ".class");
InputStream is = jar.getInputStream(entry);
ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
int nextValue = is.read();
while (-1 != nextValue) {
byteStream.write(nextValue);
nextValue = is.read();
}
classByte = byteStream.toByteArray();
result = defineClass(className, classByte, 0, classByte.length, null);
classes.put(className, result);
} catch (Exception e) {
continue;
}
}
result = (Class) classes.get(className);
if (result != null) {
return result;
}
else{
throw new ClassNotFoundException("Not found "+ className);
}
}
catch( ClassNotFoundException e ){
System.out.println("Delegating to parent : " + className);
// didn't find it, try the parent
return super.loadClass(className, resolve);
}
}
}
ParentLastClassLoader 实例化如下。
ClassLoader loader = new ParentLastClassLoader(Thread.currentThread().getContextClassLoader(), paths);
实例化ParentLastClassLoader后，将加载 MainClassOfTheFeature 并将其<将调用strong> MainMethod 。
Once the ParentLastClassLoader is instantiated, the MainClassOfTheFeature will be loaded and its MainMethod will be invoked.
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