声明： 不是很有空，慢慢完善

1. 若创建的类的实例需要较大的灵活性，则通过反射来进行创建

2. 创建的类需要用指定的类载入器载入进来

3. new URL().openConnection() 若url为http类型的，则实际上就是新建一个HttpURLConnection对象

4. URLConnection对象的getInputStream()方法就是得到HttpURLConnection对象的InputStream成员

5. IO流的打开和关闭对性能影响不大，但是不应同时拥有过多IO流对象

/**

* method in class java.net.URL

*/

static URLStreamHandler getURLStreamHandler(String protocol) {

URLStreamHandler handler = (URLStreamHandler)handlers.get(protocol);

if (handler == null) {

boolean checkedWithFactory = false;

// Use the factory (if any)

if (factory != null) {

handler = factory.createURLStreamHandler(protocol);

checkedWithFactory = true;

}

// Try java protocol handler

if (handler == null) {

String packagePrefixList = null;

packagePrefixList

= java.security.AccessController.doPrivileged(

new sun.security.action.GetPropertyAction(

protocolPathProp,""));

if (packagePrefixList != "") {

packagePrefixList += "|";

}

// REMIND: decide whether to allow the "null" class prefix

// or not.

packagePrefixList += JDK_PACKAGE_PREFIX;

StringTokenizer packagePrefixIter =

new StringTokenizer(packagePrefixList, "|");

while (handler == null &&

packagePrefixIter.hasMoreTokens()) {

String packagePrefix =

packagePrefixIter.nextToken().trim();

// do not try to instantiate the JDK gopher handler

// unless the system property had been explicitly set

if (protocol.equalsIgnoreCase(GOPHER) &&

packagePrefix.equals(JDK_PACKAGE_PREFIX) &&

!enableGopher) {

continue;

}

try {

String clsName = packagePrefix + "." + protocol +

".Handler";

Class cls = null;

try {

cls = Class.forName(clsName);

} catch (ClassNotFoundException e) {

ClassLoader cl = ClassLoader.getSystemClassLoader();

if (cl != null) {

cls = cl.loadClass(clsName);

}

}

if (cls != null) {

handler =

(URLStreamHandler)cls.newInstance();

}

} catch (Exception e) {

// any number of exceptions can get thrown here

}

}

}

synchronized (streamHandlerLock) {

URLStreamHandler handler2 = null;

// Check again with hashtable just in case another

// thread created a handler since we last checked

handler2 = (URLStreamHandler)handlers.get(protocol);

if (handler2 != null) {

return handler2;

}

// Check with factory if another thread set a

// factory since our last check

if (!checkedWithFactory && factory != null) {

handler2 = factory.createURLStreamHandler(protocol);

}

if (handler2 != null) {

// The handler from the factory must be given more

// importance. Discard the default handler that

// this thread created.

handler = handler2;

}

// Insert this handler into the hashtable

if (handler != null) {

handlers.put(protocol, handler);

}

}

}

return handler;

}