类加载器-线程上下文类加载器

破坏委派模型

问题：SPI 的接口是 Java 核心库的一部分，是由引导类加载器来加载的;SPI 实现的 Java 类一般是由系统类加载器来加载的。引导类加载器是无法找到 SPI 的实现类的，因为它只加载 Java 的核心库。它也不能代理给系统类加载器，因为它是系统类加载器的祖先类加载器。也就是说，类加载器的代理模式无法解决这个问题。

线程上下文类加载器正好解决了这个问题。如果不做任何的设置，Java 应用的线程的上下文类加载器默认就是系统上下文类加载器。在 SPI 接口的代码中使用线程上下文类加载器，就可以成功的加载到 SPI 实现的类。线程上下文类加载器在很多 SPI 的实现中都会用到。

上下文类加载器为同样在J2SE中引入的类加载代理机制提供了后门。通常JVM中的类加载器是按照层次结构组织的，目的是每个类加载器（除了启动整个JVM的原初类加载器）都有一个父类加载器。当类加载请求到来时，类加载器通常首先将请求代理给父类加载器。只有当父类加载器失败后，它才试图按照自己的算法查找并定义当前类。让核心JNDI类使用线程上下文类加载器，从而有效的打通类加载器层次结构，逆着代理机制的方向使用类加载器。

例子：

DriverManager

 //  Worker method called by the public getConnection() methods.
    private static Connection getConnection(
String url, java.util.Properties info, ClassLoader callerCL) throws SQLException {
java.util.Vector drivers = null;
        /*
* When callerCl is null, we should check the application's
* (which is invoking this class indirectly)
* classloader, so that the JDBC driver class outside rt.jar
* can be loaded from here.
*/
synchronized(DriverManager.class) { 
 // synchronize loading of the correct classloader.
 if(callerCL == null) {
     callerCL = Thread.currentThread().getContextClassLoader();
  }    
} 
 
if(url == null) {
   throw new SQLException("The url cannot be null", "08001");
}
    
println("DriverManager.getConnection(\"" + url + "\")");
    
if (!initialized) {
   initialize();
}


synchronized (DriverManager.class){ 
            // use the readcopy of drivers
   drivers = readDrivers;  
        }


// Walk through the loaded drivers attempting to make a connection.
// Remember the first exception that gets raised so we can reraise it.
SQLException reason = null;
for (int i = 0; i < drivers.size(); i++) {
   DriverInfo di = (DriverInfo)drivers.elementAt(i);
      
   // If the caller does not have permission to load the driver then 
   // skip it.
   if ( getCallerClass(callerCL, di.driverClassName ) != di.driverClass ) {
println("    skipping: " + di);
continue;
   }
   try {
println("    trying " + di);
Connection result = di.driver.connect(url, info);
if (result != null) {
   // Success!
   println("getConnection returning " + di);
   return (result);
}
   } catch (SQLException ex) {
if (reason == null) {
   reason = ex;
}
   }
}

private static ClassgetCallerClass(ClassLoader callerClassLoader, 
String driverClassName) {
Class callerC = null;


try {
   callerC = Class.forName(driverClassName, true, callerClassLoader);
}
catch (Exception ex) {
   callerC = null;           // being very careful 
}


return callerC;
    }

如红色标注：DriverManager由BootstrapLoader加载，调用的Driver具体实现由线程上下文类加载器加载，BootstrapLoader调用线程上下文类加载器，反过来，破坏了委托模型，“打通”