Thread.currentThread().setContextClassLoader为什么不生效与java.lang.NoClassDefFoundError之Java类加载的Parent fir...

 1 #### NoClassDefFoundErrorSimulator.java
 2 package org.ph.javaee.training3;
 3 
 4 import java.net.URL;
 5 import java.net.URLClassLoader;
 6 
 7 import org.ph.javaee.training.util.JavaEETrainingUtil;
 8 
 9 /**
10  * NoClassDefFoundErrorSimulator
11  * @author Pierre-Hugues Charbonneau
12  *
13  */
14 public class NoClassDefFoundErrorSimulator {
15        
16         /**
17          * @param args
18          */
19         public static void main(String[] args) {
20               
21                System.out.println("java.lang.NoClassDefFoundError Simulator - Training 3");
22                System.out.println("Author: Pierre-Hugues Charbonneau");
23                System.out.println("http://javaeesupportpatterns.blogspot.com");
24               
25                // Local variables
26                String currentThreadName = Thread.currentThread().getName();
27                String callerFullClassName = "org.ph.javaee.training3.CallerClassA";
28               
29                // Print current ClassLoader context & Thread
30                System.out.println("\nCurrent Thread name: '"+currentThreadName+"'");
31                System.out.println("Initial ClassLoader chain: "+JavaEETrainingUtil.getCurrentClassloaderDetail());
32               
33                try {
34                        // Location of the application code for our child ClassLoader
35                        URL[] webAppLibURL = new URL[] {new URL("file:caller.jar"),new URL("file:referencer.jar")};
36                       
37                        // Child ClassLoader instance creation              
38                        URLClassLoader childClassLoader = new URLClassLoader(webAppLibURL);
39                       
40                        /*** Application code execution... ***/
41                       
42                        // 1. Change the current Thread ClassLoader to the child ClassLoader
43                        Thread.currentThread().setContextClassLoader(childClassLoader);
44                        System.out.println(">> Thread '"+currentThreadName+"' Context ClassLoader now changed to '"+childClassLoader+"'");
45                        System.out.println("\nNew ClassLoader chain: "+JavaEETrainingUtil.getCurrentClassloaderDetail());
46                       
47                        // 2. Load the caller Class within the child ClassLoader...
48                        System.out.println(">> Loading '"+callerFullClassName+"' to child ClassLoader '"+childClassLoader+"'...");
49                        Class<?> callerClass = childClassLoader.loadClass(callerFullClassName);
50                       
51                        // 3. Create a new instance of CallerClassA
52                        Object callerClassInstance = callerClass.newInstance();                    
53                       
54                } catch (Throwable any) {
55                        System.out.println("Throwable: "+any);
56                        any.printStackTrace();
57                }
58               
59                System.out.println("\nSimulator completed!");
60         }
61 }  

 1 #### CallerClassA.java
 2 package org.ph.javaee.training3;
 3 
 4 import org.ph.javaee.training3.ReferencingClassA;
 5 
 6 /**
 7  * CallerClassA
 8  * @author Pierre-Hugues Charbonneau
 9  *
10  */
11 public class CallerClassA {
12        
13         private final static Class<CallerClassA> CLAZZ = CallerClassA.class;
14        
15         static {
16                System.out.println("Class loading of "+CLAZZ+" from ClassLoader '"+CLAZZ.getClassLoader()+"' in progress...");
17         }
18        
19         public CallerClassA() {
20                System.out.println("Creating a new instance of "+CallerClassA.class.getName()+"...");
21               
22                doSomething();
23         }
24        
25         private void doSomething() {
26               
27                // Create a new instance of ReferencingClassA
28                ReferencingClassA referencingClass = new ReferencingClassA();             
29         }
30 }  

 1 #### ReferencingClassA.java
 2 package org.ph.javaee.training3;
 3 
 4 /**
 5  * ReferencingClassA
 6  * @author Pierre-Hugues Charbonneau
 7  *
 8  */
 9 public class ReferencingClassA {
10        
11         private final static Class<ReferencingClassA> CLAZZ = ReferencingClassA.class;
12        
13         static {
14                System.out.println("Class loading of "+CLAZZ+" from ClassLoader '"+CLAZZ.getClassLoader()+"' in progress...");
15         }
16        
17         public ReferencingClassA() {
18                System.out.println("Creating a new instance of "+ReferencingClassA.class.getName()+"...");
19         }
20        
21         public void doSomething() {
22                //nothing to do...
23         }
24 }  

问题重现

 

为了复现问题，简单的将caller和referencing分别打包并且赋给不同的加载器。现在，可以先以正确的jar包部署来执行程序： 

 1 ## Baseline (normal execution)
 2 <JDK_HOME>\bin>java -classpath MainProgram.jar org.ph.javaee.training3.NoClassDefFoundErrorSimulator
 3 
 4 java.lang.NoClassDefFoundError Simulator - Training 3
 5 Author: Pierre-Hugues Charbonneau
 6 http://javaeesupportpatterns.blogspot.com
 7 
 8 Current Thread name: 'main'
 9 Initial ClassLoader chain:
10 -----------------------------------------------------------------
11 sun.misc.Launcher$ExtClassLoader@17c1e333
12 --- delegation ---
13 sun.misc.Launcher$AppClassLoader@214c4ac9 **Current Thread 'main' Context ClassLoader**
14 -----------------------------------------------------------------
15 
16 >> Thread 'main' Context ClassLoader now changed to 'java.net.URLClassLoader@6a4d37e5'
17 
18 New ClassLoader chain:
19 -----------------------------------------------------------------
20 sun.misc.Launcher$ExtClassLoader@17c1e333
21 --- delegation ---
22 sun.misc.Launcher$AppClassLoader@214c4ac9
23 --- delegation ---
24 java.net.URLClassLoader@6a4d37e5 **Current Thread 'main' Context ClassLoader**
25 -----------------------------------------------------------------
26 
27 >> Loading 'org.ph.javaee.training3.CallerClassA' to child ClassLoader 'java.net.URLClassLoader@6a4d37e5'...
28 Class loading of class org.ph.javaee.training3.CallerClassA from ClassLoader 'java.net.URLClassLoader@6a4d37e5' in progress...
29 Creating a new instance of org.ph.javaee.training3.CallerClassA...
30 Class loading of class org.ph.javaee.training3.ReferencingClassA from ClassLoader 'java.net.URLClassLoader@6a4d37e5' in progress...
31 Creating a new instance of org.ph.javaee.training3.ReferencingClassA...
32 
33 Simulator completed!  

在基准测试中，主程序可以正常运行，CallerClassA和他引用的类都可以被加载和实例化。

 

现在再次尝试运行程序，但是以一种错误的打包和部署方式来进行：

 1 ## Problem reproduction run (static variable initializer failure)
 2 <JDK_HOME>\bin>java -classpath MainProgram.jar;caller.jar org.ph.javaee.training3.NoClassDefFoundErrorSimulator
 3 
 4 java.lang.NoClassDefFoundError Simulator - Training 3
 5 Author: Pierre-Hugues Charbonneau
 6 http://javaeesupportpatterns.blogspot.com
 7 
 8 Current Thread name: 'main'
 9 Initial ClassLoader chain:
10 -----------------------------------------------------------------
11 sun.misc.Launcher$ExtClassLoader@17c1e333
12 --- delegation ---
13 sun.misc.Launcher$AppClassLoader@214c4ac9 **Current Thread 'main' Context ClassLoader**
14 -----------------------------------------------------------------
15 
16 >> Thread 'main' Context ClassLoader now changed to 'java.net.URLClassLoader@6a4d37e5'
17 
18 New ClassLoader chain:
19 -----------------------------------------------------------------
20 sun.misc.Launcher$ExtClassLoader@17c1e333
21 --- delegation ---
22 sun.misc.Launcher$AppClassLoader@214c4ac9
23 --- delegation ---
24 java.net.URLClassLoader@6a4d37e5 **Current Thread 'main' Context ClassLoader**
25 -----------------------------------------------------------------
26 
27 >> Loading 'org.ph.javaee.training3.CallerClassA' to child ClassLoader 'java.net.URLClassLoader@6a4d37e5'...
28 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???
29 Creating a new instance of org.ph.javaee.training3.CallerClassA...
30 Throwable: java.lang.NoClassDefFoundError: org/ph/javaee/training3/ReferencingClassA
31 java.lang.NoClassDefFoundError: org/ph/javaee/training3/ReferencingClassA
32         at org.ph.javaee.training3.CallerClassA.doSomething(CallerClassA.java:27)
33         at org.ph.javaee.training3.CallerClassA.<init>(CallerClassA.java:21)
34         at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
35         at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
36         at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
37         at java.lang.reflect.Constructor.newInstance(Unknown Source)
38         at java.lang.Class.newInstance0(Unknown Source)
39         at java.lang.Class.newInstance(Unknown Source)
40         at org.ph.javaee.training3.NoClassDefFoundErrorSimulator.main(NoClassDefFoundErrorSimulator.java:51)
41 Caused by: java.lang.ClassNotFoundException: org.ph.javaee.training3.ReferencingClassA
42         at java.net.URLClassLoader$1.run(Unknown Source)
43         at java.net.URLClassLoader$1.run(Unknown Source)
44         at java.security.AccessController.doPrivileged(Native Method)
45         at java.net.URLClassLoader.findClass(Unknown Source)
46         at java.lang.ClassLoader.loadClass(Unknown Source)
47         at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
48         at java.lang.ClassLoader.loadClass(Unknown Source)
49         ... 9 more
50 
51 Simulator completed!  

发生什么了?

 

 

关键点在于理解为什么CallerClassA会被父加载器加载。答案就在于默认的类加载委派模型，尽管子加载器和父加载器都包含caller的jar包，但默认的委派模型是父优先，这就导致caller只会在父加载器中被加载。但同时caller引用的ReferencingClassA却被部署在了子加载器，所以java.lang.NoClassDefFoundError自然会发生。

 正如你所见，由于默认的类加载委托模型的存在，代码的打包或者第三方的api都会导致这个问题的发生。所以，很重要的事情就是，你需要检视你的类加载器链条，确定是否有相同的代码或类库被重复部署在不同的父加载器和子加载器中。

 

建议和策略

 

如下是给出的建议和策略：