Thread.currentThread().setContextClassLoader为什么不生效与java.lang.NoClassDefFoundError之Java类加载的Parent first Classloader
- JVM将程序的部分类型在父加载器中加载了(比如系统或者父加载器)
- JVM将程序的其他部分想要在子加载器中加载(比如容器或者用户自定义加载器)
- 主程序NoClassDefFoundErrorSimulator会被打包在MainProgram.jar
- 日志输出程序JavaEETrainingUtil也被打包在MainProgram.jar
- caller类CallerClassA被打包在caller.jar
- 被引用的类ReferencingClassA被打包在referencer.jar
- 创建一个子加载器(java.net.URLClassLoader)
- 将caller.jar和referencher.jar分配给上面创建的子加载器
- 将当前线程上下文加载器更改为上面的子加载器
- 尝试从当前线程上下文加载器加载和创建CallerClassA类的实例
- 日志输出用来帮助理解类加载器的变化和线程上下文加载器的状态
#### NoClassDefFoundErrorSimulator.java
package org.ph.javaee.training3; import java.net.URL;
import java.net.URLClassLoader; import org.ph.javaee.training.util.JavaEETrainingUtil; /**
* NoClassDefFoundErrorSimulator
* @author Pierre-Hugues Charbonneau
*
*/
public class NoClassDefFoundErrorSimulator { /**
* @param args
*/
public static void main(String[] args) { System.out.println("java.lang.NoClassDefFoundError Simulator - Training 3");
System.out.println("Author: Pierre-Hugues Charbonneau");
System.out.println("http://javaeesupportpatterns.blogspot.com"); // Local variables
String currentThreadName = Thread.currentThread().getName();
String callerFullClassName = "org.ph.javaee.training3.CallerClassA"; // Print current ClassLoader context & Thread
System.out.println("\nCurrent Thread name: '"+currentThreadName+"'");
System.out.println("Initial ClassLoader chain: "+JavaEETrainingUtil.getCurrentClassloaderDetail()); try {
// Location of the application code for our child ClassLoader
URL[] webAppLibURL = new URL[] {new URL("file:caller.jar"),new URL("file:referencer.jar")}; // Child ClassLoader instance creation
URLClassLoader childClassLoader = new URLClassLoader(webAppLibURL); /*** Application code execution... ***/ // 1. Change the current Thread ClassLoader to the child ClassLoader
Thread.currentThread().setContextClassLoader(childClassLoader);
System.out.println(">> Thread '"+currentThreadName+"' Context ClassLoader now changed to '"+childClassLoader+"'");
System.out.println("\nNew ClassLoader chain: "+JavaEETrainingUtil.getCurrentClassloaderDetail()); // 2. Load the caller Class within the child ClassLoader...
System.out.println(">> Loading '"+callerFullClassName+"' to child ClassLoader '"+childClassLoader+"'...");
Class<?> callerClass = childClassLoader.loadClass(callerFullClassName); // 3. Create a new instance of CallerClassA
Object callerClassInstance = callerClass.newInstance(); } catch (Throwable any) {
System.out.println("Throwable: "+any);
any.printStackTrace();
} System.out.println("\nSimulator completed!");
}
}
#### CallerClassA.java
package org.ph.javaee.training3; import org.ph.javaee.training3.ReferencingClassA; /**
* CallerClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class CallerClassA { private final static Class<CallerClassA> CLAZZ = CallerClassA.class; static {
System.out.println("Class loading of "+CLAZZ+" from ClassLoader '"+CLAZZ.getClassLoader()+"' in progress...");
} public CallerClassA() {
System.out.println("Creating a new instance of "+CallerClassA.class.getName()+"..."); doSomething();
} private void doSomething() { // Create a new instance of ReferencingClassA
ReferencingClassA referencingClass = new ReferencingClassA();
}
}
#### ReferencingClassA.java
package org.ph.javaee.training3; /**
* ReferencingClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class ReferencingClassA { private final static Class<ReferencingClassA> CLAZZ = ReferencingClassA.class; static {
System.out.println("Class loading of "+CLAZZ+" from ClassLoader '"+CLAZZ.getClassLoader()+"' in progress...");
} public ReferencingClassA() {
System.out.println("Creating a new instance of "+ReferencingClassA.class.getName()+"...");
} public void doSomething() {
//nothing to do...
}
}
- 主程序和工具包被部署在父加载器 (SYSTEM classpath)
- CallerClassA 和ReferencingClassA被部署在子加载器中
## Baseline (normal execution)
<JDK_HOME>\bin>java -classpath MainProgram.jar org.ph.javaee.training3.NoClassDefFoundErrorSimulator java.lang.NoClassDefFoundError Simulator - Training 3
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com Current Thread name: 'main'
Initial ClassLoader chain:
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9 **Current Thread 'main' Context ClassLoader**
----------------------------------------------------------------- >> Thread 'main' Context ClassLoader now changed to 'java.net.URLClassLoader@6a4d37e5' New ClassLoader chain:
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9
--- delegation ---
java.net.URLClassLoader@6a4d37e5 **Current Thread 'main' Context ClassLoader**
----------------------------------------------------------------- >> Loading 'org.ph.javaee.training3.CallerClassA' to child ClassLoader 'java.net.URLClassLoader@6a4d37e5'...
Class loading of class org.ph.javaee.training3.CallerClassA from ClassLoader 'java.net.URLClassLoader@6a4d37e5' in progress...
Creating a new instance of org.ph.javaee.training3.CallerClassA...
Class loading of class org.ph.javaee.training3.ReferencingClassA from ClassLoader 'java.net.URLClassLoader@6a4d37e5' in progress...
Creating a new instance of org.ph.javaee.training3.ReferencingClassA... Simulator completed!
- 主程序和工具类都被部署在父加载器(SYSTEM classpath)
- CallerClassA和ReferencingClassA被部署在子加载器
- CallerClassA (caller.jar)也被部署在父加载器
## Problem reproduction run (static variable initializer failure)
<JDK_HOME>\bin>java -classpath MainProgram.jar;caller.jar org.ph.javaee.training3.NoClassDefFoundErrorSimulator java.lang.NoClassDefFoundError Simulator - Training 3
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com Current Thread name: 'main'
Initial ClassLoader chain:
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9 **Current Thread 'main' Context ClassLoader**
----------------------------------------------------------------- >> Thread 'main' Context ClassLoader now changed to 'java.net.URLClassLoader@6a4d37e5' New ClassLoader chain:
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9
--- delegation ---
java.net.URLClassLoader@6a4d37e5 **Current Thread 'main' Context ClassLoader**
----------------------------------------------------------------- >> Loading 'org.ph.javaee.training3.CallerClassA' to child ClassLoader 'java.net.URLClassLoader@6a4d37e5'...
Class loading of class org.ph.javaee.training3.CallerClassA from ClassLoader 'sun.misc.Launcher$AppClassLoader@214c4ac9' in progress...// Caller is loaded from the parent class loader, why???
Creating a new instance of org.ph.javaee.training3.CallerClassA...
Throwable: java.lang.NoClassDefFoundError: org/ph/javaee/training3/ReferencingClassA
java.lang.NoClassDefFoundError: org/ph/javaee/training3/ReferencingClassA
at org.ph.javaee.training3.CallerClassA.doSomething(CallerClassA.java:27)
at org.ph.javaee.training3.CallerClassA.<init>(CallerClassA.java:21)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at java.lang.Class.newInstance0(Unknown Source)
at java.lang.Class.newInstance(Unknown Source)
at org.ph.javaee.training3.NoClassDefFoundErrorSimulator.main(NoClassDefFoundErrorSimulator.java:51)
Caused by: java.lang.ClassNotFoundException: org.ph.javaee.training3.ReferencingClassA
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 9 more Simulator completed!
- 主程序和工具类都正常的从父加载器加载成功 (sun.misc.Launcher$AppClassLoader)
- 线程上下文加载器被更改为包含caller和reference的子加载器。
- 但是我们会看到CallerClassA 会被父加载器加载 (sun.misc.Launcher$AppClassLoader) ,并没有被子加载器加载
- 由于ReferencingClassA 没有被部署在父加载器, 所以他不能在当前加载器中加载,又由于父加载器对子加载器的无知,所以也不可能被子加载器加载。然后就报错NoClassDefFoundError。

- 仔细检视异常java.lang.NoClassDefFoundError并确定到底是哪个类导致问题
- 检视受影响的程序的打包方式,看看是否能找到是相应的类存在重复或者错误的部署方式。(SYSTEM class path, EAR file, Java EE container itself etc.).
- 如果找到了,那就需要将响应的类库从受影响的类加载器中移除。(有时会很复杂)
- 启用jvm的属性verbose,比如–verbose:class很有帮助,会帮助你定位类是从哪个加载器中加载的。
翻译自:https://javaeesupportpatterns.blogspot.com/2012/08/javalangnoclassdeffounderror-parent.html
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