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

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       maven-assembly-plugin
                      
      
       2.4
                      
                          
                               
        
         jar-with-dependencies
                            
                           
                               
                                    
         
          Main.Main
                                 
                            
                       
                      
                          
                               
        
         make-assembly
                                
        
         package
                                
                                    
         
          single
                                 
                            
                       
                  
     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

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       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

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     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;        }    }

4.mvn jar包冲突常用命令

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  mvn dependency:analyze，mvn dependency:tree