java url api_深入阅读java api之URL
聲明： 不是很有空，慢慢完善
1. 若創建的類的實例需要較大的靈活性，則通過反射來進行創建
2. 創建的類需要用指定的類載入器載入進來
3. new URL().openConnection() 若url為http類型的，則實際上就是新建一個HttpURLConnection對象
4. URLConnection對象的getInputStream()方法就是得到HttpURLConnection對象的InputStream成員
5. IO流的打開和關閉對性能影響不大，但是不應同時擁有過多IO流對象
/**
* 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;
}
總結
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