ClassLoader、ContextClassLoader加载问题（转）

本文的目的重温ClassLoader、ContextClassLoader加载使用，展示可能忽略的东西，虽然内容不是多么复杂，但是出现一些问题而不明白原因时，会让人"抓狂"。

public class ConfigurationPathTest {
       public static void main(String[] arr){
           ClassLoader cload = Thread.currentThread().getContextClassLoader();
           System.out.println("------------------getContextClassLoader加载指定的common/conf/configuration.xml配置文件---------------------"+cload.getResource("common/conf/configuration.xml"));
           //StandaloneContainer exo.kernel.container-2.1.2.jar
           System.out.println("---------------------getClassLoader加载指定的common/conf/configuration.xml配置文件--------------------------------:"+ConfigurationPathTest.class.getClassLoader().getResource("common/conf/configuration.xml"));
           //jar:file:/D:/workspace/ExoDemo/lib/exo.kernel.container-2.1.2.jar!/conf/configuration.xml
           //-----------------------具体类路径和classpath路径--------------------------//
           System.out.println("-------------getClassLoader加载classpath----------------"+ConfigurationPathTest.class.getResource(""));
           System.out.println("-------------getContextClassLoader加载classpath----------------"+cload.getResource(""));
           System.out.println("-------------System classpath----------------"+ConfigurationPathTest.class.getClassLoader().getResource(""));

           //-----------------------优先从classpath加载，然后从lib的jar包加载--------------------------//
           System.out.println("-------------第二次 getClassLoader加载classpath----------------"+ConfigurationPathTest.class.getResource("/login.conf"));
           System.out.println("-------------第二次getContextClassLoader加载classpath ----------------"+cload.getResource("login.conf"));
           System.out.println("-------------第二次System classpath ----------------"+ConfigurationPathTest.class.getClassLoader().getResource("login.conf"));

           System.out.println("-------------第3次getClassLoader加载classpath----------------"+ConfigurationPathTest.class.getResource("/login_prev.conf"));
           System.out.println("-------------第3次getContextClassLoader加载classpath----------------"+cload.getResource("login_prev.conf"));
           System.out.println("-------------第3次System classpath----------------"+ConfigurationPathTest.class.getClassLoader().getResource("login_prev.conf"));

       }

}

public class ConfigurationPathTest {
       public static void main(String[] arr){
           ClassLoader cload = Thread.currentThread().getContextClassLoader();    
           System.out.println("------------------getContextClassLoader加载指定的common/conf/configuration.xml配置文件---------------------"+cload.getResource("common/conf/configuration.xml"));
           //StandaloneContainer exo.kernel.container-2.1.2.jar
           System.out.println("---------------------getClassLoader加载指定的common/conf/configuration.xml配置文件--------------------------------:"+ConfigurationPathTest.class.getClassLoader().getResource("common/conf/configuration.xml"));
           //jar:file:/D:/workspace/ExoDemo/lib/exo.kernel.container-2.1.2.jar!/conf/configuration.xml
           //-----------------------具体类路径和classpath路径--------------------------//           
           System.out.println("-------------getClassLoader加载classpath----------------"+ConfigurationPathTest.class.getResource(""));         
           System.out.println("-------------getContextClassLoader加载classpath----------------"+cload.getResource(""));           
           System.out.println("-------------System classpath----------------"+ConfigurationPathTest.class.getClassLoader().getResource(""));
           
           //-----------------------优先从classpath加载，然后从lib的jar包加载--------------------------//
           System.out.println("-------------第二次 getClassLoader加载classpath----------------"+ConfigurationPathTest.class.getResource("/login.conf"));         
           System.out.println("-------------第二次getContextClassLoader加载classpath ----------------"+cload.getResource("login.conf"));           
           System.out.println("-------------第二次System classpath ----------------"+ConfigurationPathTest.class.getClassLoader().getResource("login.conf"));
           
           System.out.println("-------------第3次getClassLoader加载classpath----------------"+ConfigurationPathTest.class.getResource("/login_prev.conf"));         
           System.out.println("-------------第3次getContextClassLoader加载classpath----------------"+cload.getResource("login_prev.conf"));           
           System.out.println("-------------第3次System classpath----------------"+ConfigurationPathTest.class.getClassLoader().getResource("login_prev.conf"));
       
       }
    
}

执行，打印内容关键罗列如下：

file:/D:/workspace/UCMTest/bin/com/huaweisymantec/ucm/jcr/api/
file:/D:/workspace/UCMTest/bin/
file:/D:/workspace/UCMTest/bin/

file:/D:/workspace/UCMTest/bin/login.conf
file:/D:/workspace/UCMTest/bin/login.conf
file:/D:/workspace/UCMTest/bin/login.conf

jar:file:/D:/workspace/UCMTest/lib/com.huaweisymantec.ucm-1.0.jar!/login_prev.conf
jar:file:/D:/workspace/UCMTest/lib/com.huaweisymantec.ucm-1.0.jar!/login_prev.conf
jar:file:/D:/workspace/UCMTest/lib/com.huaweisymantec.ucm-1.0.jar!/login_prev.conf

file:/D:/workspace/UCMTest/bin/com/huaweisymantec/ucm/jcr/api/
file:/D:/workspace/UCMTest/bin/
file:/D:/workspace/UCMTest/bin/

file:/D:/workspace/UCMTest/bin/login.conf
file:/D:/workspace/UCMTest/bin/login.conf
file:/D:/workspace/UCMTest/bin/login.conf

jar:file:/D:/workspace/UCMTest/lib/com.huaweisymantec.ucm-1.0.jar!/login_prev.conf
jar:file:/D:/workspace/UCMTest/lib/com.huaweisymantec.ucm-1.0.jar!/login_prev.conf
jar:file:/D:/workspace/UCMTest/lib/com.huaweisymantec.ucm-1.0.jar!/login_prev.conf

从加载路径可以看出
(1)ConfigurationPathTest.class.getResource("")是指明了具体包路径
而ClassLoader.getResource("")指明了classpath路径
(2)同样是从classpath加载，写法有一点点区别（要小心）
ConfigurationPathTest.class.getResource("/login.conf")       
ClassLoader.getResource("login.conf")
(3)getResource()方法会首先从classpath路径去找要加载的文件；如果不存在，再去jar包加载；如果都没有，返回null
(4)getResources()方法同时加载classpath和jar内的内容，如:ClassLoader.getResources("common/conf/configuration.xml");
（如果class和jar中都有此配置文件,使用不当会造成困惑：两个都加载了，到底哪个是在使用的；如果出错，哪个导致了出错）