Classloader Error: attempted duplicate class definition for name

Problem

在寫plugin相關程式時，會去override classloader做成自己想要的載入形式。而這陣子，偶爾會發生如以下Error:

java.lang.LinkageError: loader (instance of  org/tonylin/FunnyClassLoader): attempted  duplicate class definition for name: "org/apache/commons/cli/BasicParser"

而我們的實做是去extend java.net.URLClassLoader，其中關鍵程式碼如下:

public Class<?> loadClass(String className) throws ClassNotFoundException {
	Class<?> clazz = findLoadedClass(className);
	if (clazz != null) {
		return clazz;
	}
 
	// load locally
	try {
		clazz = findClass(className);
		return clazz;
	} catch (ClassNotFoundException e) {
		// ignore
	}
 
	// use the standard URLClassLoader (which follows normal parent
	// delegation)
	try {
		return super.loadClass(className);
	} catch (ClassNotFoundException e) {
		return null;
	}
}

目的是為了先讀取已載入的class，若尚未載入則優先搜尋classpath中是否有此class，最後才去找parent。

Solutions

我寫了一個簡單的單元測試去trace這個問題，而內容為:

• 新增一個jar檔到FunnyClassLoader的classpath中。
• 產生5個thread去load某個jar檔中的class。
• 驗證正確完成的工作數量為5。

@Test
public void testConcurrentLoad() throws Exception{
	mClassLoader = new FunnyClassLoader(Thread.currentThread().getContextClassLoader());
	mClassLoader.addURL(new File("./testdata/commons-cli-1.2.jar"));
 
 
	List<Thread> threads = new ArrayList<>();
	List<String> passThread = new ArrayList<>();
	int size = 5;
	CountDownLatch latch = new CountDownLatch(size);
 
	for( int i = 0 ; i < size ; i ++ ){
 
		Thread t = new Thread(()->{
			try {
				mClassLoader.loadClass("org.apache.commons.cli.BasicParser");
				System.out.println("pass: " + Thread.currentThread().getName());
				passThread.add(Thread.currentThread().getName());
			} catch (Exception e) {
				e.printStackTrace();
			} finally {
				latch.countDown();
			}
		});
		threads.add(t);
	}
	threads.parallelStream().forEach(t->t.start());
 
	latch.await();
 
	assertEquals(threads.size(), passThread.size());
}

最後發現問題是由於: 當兩個thread同時存取loadclass時，若class已被define過，就會產生此錯誤。最後寫法是:

@Override
public Class<?> loadClass(String className) throws ClassNotFoundException {
   synchronized (getClassLoadingLock(className)) {
        // original code 
   }
}