问题
I'm trying to load a class from JAR represented as a byte[] array at runtime.
I know two things about class to load:
1. it implements "RequiredInterface"
2. I know it's qualified name: "sample.TestJarLoadingClass"
I found the solution in which I have to extend ClassLoader but it throws:
Exception in thread "main" java.lang.ClassNotFoundException: sample.TestJarLoadingClass
at java.lang.ClassLoader.findClass(ClassLoader.java:530)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at tasks.Main.main(Main.java:12)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
whenever I want to load the class.
What can be the reason of this situation and how can I get rid of that?
Any help highly appreciated
Main method:
public static void main(String[] args) throws IOException, ClassNotFoundException, IllegalAccessException, InstantiationException {
Path path = Paths.get("src/main/java/tasks/sample.jar");
RequiredInterface requiredInterface = (RequiredInterface) Class.forName("sample.TestJarLoadingClass", true, new ByteClassLoader(Files.readAllBytes(path))).newInstance();
}
Custom class loader:
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
import java.util.zip.ZipEntry;
import java.util.zip.ZipInputStream;
public class ByteClassLoader extends ClassLoader {
private final byte[] jarBytes;
private final Set<String> names;
public ByteClassLoader(byte[] jarBytes) throws IOException {
this.jarBytes = jarBytes;
this.names = loadNames(jarBytes);
}
private Set<String> loadNames(byte[] jarBytes) throws IOException {
Set<String> set = new HashSet<>();
try (ZipInputStream jis = new ZipInputStream(new ByteArrayInputStream(jarBytes))) {
ZipEntry entry;
while ((entry = jis.getNextEntry()) != null) {
set.add(entry.getName());
}
}
return Collections.unmodifiableSet(set);
}
@Override
public InputStream getResourceAsStream(String resourceName) {
if (!names.contains(resourceName)) {
return null;
}
boolean found = false;
ZipInputStream zipInputStream = null;
try {
zipInputStream = new ZipInputStream(new ByteArrayInputStream(jarBytes));
ZipEntry entry;
while ((entry = zipInputStream.getNextEntry()) != null) {
if (entry.getName().equals(resourceName)) {
found = true;
return zipInputStream;
}
}
} catch (IOException e) {;
e.printStackTrace();
} finally {
if (zipInputStream != null && !found) {
try {
zipInputStream.close();
} catch (IOException e) {;
e.printStackTrace();
}
}
}
return null;
}
}
RequiredInterface:
public interface RequiredInterface {
String method();
}
Class in JAR file:
package sample;
public class TestJarLoadingClass implements RequiredInterface {
@Override
public String method() {
return "works!";
}
}
回答1:
In my opinion we have two problems here:
First of all you should override findClass method that contains actual logic of loading class. The main challenge here is to find part of byte array that contains your class - since you have whole jar as a byte array, you will need to use JarInputStream to scan your byte array for your class.
But this might not be enough because your RequiredInterface is unknown for your ByteClassLoader - so you will be able to read class itself, but the definition of class contains information that it implements RequiredInterface which is a problem for your class loader. This one is easy to fix, you just need to pass regular class loader as a constructor parameter to your one and use super(parentClassLoader).
Here is my version:
public class ByteClassLoader extends ClassLoader {
private final byte[] jarBytes;
public ByteClassLoader(ClassLoader parent, byte[] jarBytes) throws IOException {
super(parent);
this.jarBytes = jarBytes;
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
// read byte array with JarInputStream
try (JarInputStream jis = new JarInputStream(new ByteArrayInputStream(jarBytes))) {
JarEntry nextJarEntry;
// finding JarEntry will "move" JarInputStream at the begining of entry, so no need to create new input stream
while ((nextJarEntry = jis.getNextJarEntry()) != null) {
if (entryNameEqualsClassName(name, nextJarEntry)) {
// we need to know length of class to know how many bytes we should read
int classSize = (int) nextJarEntry.getSize();
// our buffer for class bytes
byte[] nextClass = new byte[classSize];
// actual reading
jis.read(nextClass, 0, classSize);
// create class from bytes
return defineClass(name, nextClass, 0, classSize, null);
}
}
throw new ClassNotFoundException(String.format("Cannot find %s class", name));
} catch (IOException e) {
throw new ClassNotFoundException("Cannot read from jar input stream", e);
}
}
private boolean entryNameEqualsClassName(String name, JarEntry nextJarEntry) {
// removing .class suffix
String entryName = nextJarEntry.getName().split("\\.")[0];
// "convert" fully qualified name into path
String className = name.replace(".", "/");
return entryName.equals(className);
}
}
And usage
RequiredInterface requiredInterface = (RequiredInterface)Class.forName("com.sample.TestJarLoadingClass", true, new ByteClassLoader(ByteClassLoader.class.getClassLoader(), Files.readAllBytes(path))).newInstance();
System.out.println(requiredInterface.method());
Please be aware that my implementation assumes that file name = class name, so for example classes that are not top level will not be found. And of course some details might be more polished (like exception handling).
来源：https://stackoverflow.com/questions/42325699/classnotfoundexception-while-trying-to-load-class-from-external-jar-at-runtime