浅谈两个jar包中包含完全相同的包名和类名的加载问题

首先从表现层介绍，后续后深入原理。

1、先简单介绍maven如何生成jar文件方便测试

									<plugin>

									    <artifactId>maven-assembly-plugin</artifactId>

									    <version>2.4</version>

									    <configuration>

									     <descriptorRefs>

									      <descriptorRef>jar-with-dependencies</descriptorRef>

									     </descriptorRefs>

									     <archive>

									      <manifest>

									       <mainClass>Main.Main</mainClass>

									      </manifest>

									     </archive>

									    </configuration>

									    <executions>

									     <execution>

									      <id>make-assembly</id>

									      <phase>package</phase>

									      <goals>

									       <goal>single</goal>

									      </goals>

									     </execution>

									    </executions>

									   </plugin>

									配置了一个manifest标签来配置Main函数的入口。然后通过如下指令来实现打包。

									mvn assembly:assembly

2、自定义两个jar包，其中包含相同包名和类名

与export的导入顺序有关。只会加载第一个，并且运行正常。

3、自定义jar和jdk包，其中包含相同的包名和类名

与export的导入顺序有关。同样是只会加载第一个，但是如果加载自定义的jar运行会报错。加载 jdk正常。

									protected Class<?> loadClass(String name, boolean resolve)

									  throws ClassNotFoundException

									 {

									  synchronized (getClassLoadingLock(name)) {

									   // First, check if the class has already been loaded

									   Class<?> c = findLoadedClass(name);

									   if (c == null) {

									    long t0 = System.nanoTime();

									    try {

									     if (parent != null) {

									      c = parent.loadClass(name, false);

									     } else {

									      c = findBootstrapClassOrNull(name);

									     }

									    } catch (ClassNotFoundException e) {

									     // ClassNotFoundException thrown if class not found

									     // from the non-null parent class loader

									    }

									    if (c == null) {

									     // If still not found, then invoke findClass in order

									     // to find the class.

									     long t1 = System.nanoTime();

									     c = findClass(name);

									     // this is the defining class loader; record the stats

									     sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);

									     sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);

									     sun.misc.PerfCounter.getFindClasses().increment();

									    }

									   }

									   if (resolve) {

									    resolveClass(c);

									   }

									   return c;

									  }

									 }