URLConnection.java

/*
 * Copyright (c) 1995, 2016, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.net;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Hashtable;
import java.util.Date;
import java.util.StringTokenizer;
import java.util.Collections;
import java.util.Map;
import java.util.List;
import java.security.Permission;
import java.security.AccessController;
import sun.security.util.SecurityConstants;
import sun.net.www.MessageHeader;

/**
 * The abstract class {@code URLConnection} is the superclass
 * of all classes that represent a communications link between the
 * application and a URL. Instances of this class can be used both to
 * read from and to write to the resource referenced by the URL. In
 * general, creating a connection to a URL is a multistep process:
 *
 * <center><table border=2 summary="Describes the process of creating a connection to a URL: openConnection() and connect() over time.">
 * <tr><th>{@code openConnection()}</th>
 *     <th>{@code connect()}</th></tr>
 * <tr><td>Manipulate parameters that affect the connection to the remote
 *         resource.</td>
 *     <td>Interact with the resource; query header fields and
 *         contents.</td></tr>
 * </table>
 * ----------------------------&gt;
 * <br>time</center>
 *
 * <ol>
 * <li>The connection object is created by invoking the
 *     {@code openConnection} method on a URL.
 * <li>The setup parameters and general request properties are manipulated.
 * <li>The actual connection to the remote object is made, using the
 *    {@code connect} method.
 * <li>The remote object becomes available. The header fields and the contents
 *     of the remote object can be accessed.
 * </ol>
 * <p>
 * The setup parameters are modified using the following methods:
 * <ul>
 *   <li>{@code setAllowUserInteraction}
 *   <li>{@code setDoInput}
 *   <li>{@code setDoOutput}
 *   <li>{@code setIfModifiedSince}
 *   <li>{@code setUseCaches}
 * </ul>
 * <p>
 * and the general request properties are modified using the method:
 * <ul>
 *   <li>{@code setRequestProperty}
 * </ul>
 * <p>
 * Default values for the {@code AllowUserInteraction} and
 * {@code UseCaches} parameters can be set using the methods
 * {@code setDefaultAllowUserInteraction} and
 * {@code setDefaultUseCaches}.
 * <p>
 * Each of the above {@code set} methods has a corresponding
 * {@code get} method to retrieve the value of the parameter or
 * general request property. The specific parameters and general
 * request properties that are applicable are protocol specific.
 * <p>
 * The following methods are used to access the header fields and
 * the contents after the connection is made to the remote object:
 * <ul>
 *   <li>{@code getContent}
 *   <li>{@code getHeaderField}
 *   <li>{@code getInputStream}
 *   <li>{@code getOutputStream}
 * </ul>
 * <p>
 * Certain header fields are accessed frequently. The methods:
 * <ul>
 *   <li>{@code getContentEncoding}
 *   <li>{@code getContentLength}
 *   <li>{@code getContentType}
 *   <li>{@code getDate}
 *   <li>{@code getExpiration}
 *   <li>{@code getLastModifed}
 * </ul>
 * <p>
 * provide convenient access to these fields. The
 * {@code getContentType} method is used by the
 * {@code getContent} method to determine the type of the remote
 * object; subclasses may find it convenient to override the
 * {@code getContentType} method.
 * <p>
 * In the common case, all of the pre-connection parameters and
 * general request properties can be ignored: the pre-connection
 * parameters and request properties default to sensible values. For
 * most clients of this interface, there are only two interesting
 * methods: {@code getInputStream} and {@code getContent},
 * which are mirrored in the {@code URL} class by convenience methods.
 * <p>
 * More information on the request properties and header fields of
 * an {@code http} connection can be found at:
 * <blockquote><pre>
 * <a href="http://www.ietf.org/rfc/rfc2616.txt">http://www.ietf.org/rfc/rfc2616.txt</a>
 * </pre></blockquote>
 *
 * Invoking the {@code close()} methods on the {@code InputStream} or {@code OutputStream} of an
 * {@code URLConnection} after a request may free network resources associated with this
 * instance, unless particular protocol specifications specify different behaviours
 * for it.
 *
 * @author  James Gosling
 * @see     java.net.URL#openConnection()
 * @see     java.net.URLConnection#connect()
 * @see     java.net.URLConnection#getContent()
 * @see     java.net.URLConnection#getContentEncoding()
 * @see     java.net.URLConnection#getContentLength()
 * @see     java.net.URLConnection#getContentType()
 * @see     java.net.URLConnection#getDate()
 * @see     java.net.URLConnection#getExpiration()
 * @see     java.net.URLConnection#getHeaderField(int)
 * @see     java.net.URLConnection#getHeaderField(java.lang.String)
 * @see     java.net.URLConnection#getInputStream()
 * @see     java.net.URLConnection#getLastModified()
 * @see     java.net.URLConnection#getOutputStream()
 * @see     java.net.URLConnection#setAllowUserInteraction(boolean)
 * @see     java.net.URLConnection#setDefaultUseCaches(boolean)
 * @see     java.net.URLConnection#setDoInput(boolean)
 * @see     java.net.URLConnection#setDoOutput(boolean)
 * @see     java.net.URLConnection#setIfModifiedSince(long)
 * @see     java.net.URLConnection#setRequestProperty(java.lang.String, java.lang.String)
 * @see     java.net.URLConnection#setUseCaches(boolean)
 * @since   JDK1.0
 */
public abstract class URLConnection {

   /**
     * The URL represents the remote object on the World Wide Web to
     * which this connection is opened.
     * <p>
     * The value of this field can be accessed by the
     * {@code getURL} method.
     * <p>
     * The default value of this variable is the value of the URL
     * argument in the {@code URLConnection} constructor.
     *
     * @see     java.net.URLConnection#getURL()
     * @see     java.net.URLConnection#url
     */
    protected URL url;

   /**
     * This variable is set by the {@code setDoInput} method. Its
     * value is returned by the {@code getDoInput} method.
     * <p>
     * A URL connection can be used for input and/or output. Setting the
     * {@code doInput} flag to {@code true} indicates that
     * the application intends to read data from the URL connection.
     * <p>
     * The default value of this field is {@code true}.
     *
     * @see     java.net.URLConnection#getDoInput()
     * @see     java.net.URLConnection#setDoInput(boolean)
     */
    protected boolean doInput = true;

   /**
     * This variable is set by the {@code setDoOutput} method. Its
     * value is returned by the {@code getDoOutput} method.
     * <p>
     * A URL connection can be used for input and/or output. Setting the
     * {@code doOutput} flag to {@code true} indicates
     * that the application intends to write data to the URL connection.
     * <p>
     * The default value of this field is {@code false}.
     *
     * @see     java.net.URLConnection#getDoOutput()
     * @see     java.net.URLConnection#setDoOutput(boolean)
     */
    protected boolean doOutput = false;

    private static boolean defaultAllowUserInteraction = false;

   /**
     * If {@code true}, this {@code URL} is being examined in
     * a context in which it makes sense to allow user interactions such
     * as popping up an authentication dialog. If {@code false},
     * then no user interaction is allowed.
     * <p>
     * The value of this field can be set by the
     * {@code setAllowUserInteraction} method.
     * Its value is returned by the
     * {@code getAllowUserInteraction} method.
     * Its default value is the value of the argument in the last invocation
     * of the {@code setDefaultAllowUserInteraction} method.
     *
     * @see     java.net.URLConnection#getAllowUserInteraction()
     * @see     java.net.URLConnection#setAllowUserInteraction(boolean)
     * @see     java.net.URLConnection#setDefaultAllowUserInteraction(boolean)
     */
    protected boolean allowUserInteraction = defaultAllowUserInteraction;

    private static boolean defaultUseCaches = true;

   /**
     * If {@code true}, the protocol is allowed to use caching
     * whenever it can. If {@code false}, the protocol must always
     * try to get a fresh copy of the object.
     * <p>
     * This field is set by the {@code setUseCaches} method. Its
     * value is returned by the {@code getUseCaches} method.
     * <p>
     * Its default value is the value given in the last invocation of the
     * {@code setDefaultUseCaches} method.
     *
     * @see     java.net.URLConnection#setUseCaches(boolean)
     * @see     java.net.URLConnection#getUseCaches()
     * @see     java.net.URLConnection#setDefaultUseCaches(boolean)
     */
    protected boolean useCaches = defaultUseCaches;

   /**
     * Some protocols support skipping the fetching of the object unless
     * the object has been modified more recently than a certain time.
     * <p>
     * A nonzero value gives a time as the number of milliseconds since
     * January 1, 1970, GMT. The object is fetched only if it has been
     * modified more recently than that time.
     * <p>
     * This variable is set by the {@code setIfModifiedSince}
     * method. Its value is returned by the
     * {@code getIfModifiedSince} method.
     * <p>
     * The default value of this field is {@code 0}, indicating
     * that the fetching must always occur.
     *
     * @see     java.net.URLConnection#getIfModifiedSince()
     * @see     java.net.URLConnection#setIfModifiedSince(long)
     */
    protected long ifModifiedSince = 0;

   /**
     * If {@code false}, this connection object has not created a
     * communications link to the specified URL. If {@code true},
     * the communications link has been established.
     */
    protected boolean connected = false;

    /**
     * @since 1.5
     */
    private int connectTimeout;
    private int readTimeout;

    /**
     * @since 1.6
     */
    private MessageHeader requests;

   /**
    * @since   JDK1.1
    */
    private static FileNameMap fileNameMap;

    /**
     * @since 1.2.2
     */
    private static boolean fileNameMapLoaded = false;

    /**
     * Loads filename map (a mimetable) from a data file. It will
     * first try to load the user-specific table, defined
     * by &quot;content.types.user.table&quot; property. If that fails,
     * it tries to load the default built-in table.
     *
     * @return the FileNameMap
     * @since 1.2
     * @see #setFileNameMap(java.net.FileNameMap)
     */
    public static synchronized FileNameMap getFileNameMap() {
        if ((fileNameMap == null) && !fileNameMapLoaded) {
            fileNameMap = sun.net.www.MimeTable.loadTable();
            fileNameMapLoaded = true;
        }

        return new FileNameMap() {
            private FileNameMap map = fileNameMap;
            public String getContentTypeFor(String fileName) {
                return map.getContentTypeFor(fileName);
            }
        };
    }

    /**
     * Sets the FileNameMap.
     * <p>
     * If there is a security manager, this method first calls
     * the security manager's {@code checkSetFactory} method
     * to ensure the operation is allowed.
     * This could result in a SecurityException.
     *
     * @param map the FileNameMap to be set
     * @exception  SecurityException  if a security manager exists and its
     *             {@code checkSetFactory} method doesn't allow the operation.
     * @see        SecurityManager#checkSetFactory
     * @see #getFileNameMap()
     * @since 1.2
     */
    public static void setFileNameMap(FileNameMap map) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) sm.checkSetFactory();
        fileNameMap = map;
    }

    /**
     * Opens a communications link to the resource referenced by this
     * URL, if such a connection has not already been established.
     * <p>
     * If the {@code connect} method is called when the connection
     * has already been opened (indicated by the {@code connected}
     * field having the value {@code true}), the call is ignored.
     * <p>
     * URLConnection objects go through two phases: first they are
     * created, then they are connected.  After being created, and
     * before being connected, various options can be specified
     * (e.g., doInput and UseCaches).  After connecting, it is an
     * error to try to set them.  Operations that depend on being
     * connected, like getContentLength, will implicitly perform the
     * connection, if necessary.
     *
     * @throws SocketTimeoutException if the timeout expires before
     *               the connection can be established
     * @exception  IOException  if an I/O error occurs while opening the
     *               connection.
     * @see java.net.URLConnection#connected
     * @see #getConnectTimeout()
     * @see #setConnectTimeout(int)
     */
    abstract public void connect() throws IOException;

    /**
     * Sets a specified timeout value, in milliseconds, to be used
     * when opening a communications link to the resource referenced
     * by this URLConnection.  If the timeout expires before the
     * connection can be established, a
     * java.net.SocketTimeoutException is raised. A timeout of zero is
     * interpreted as an infinite timeout.

     * <p> Some non-standard implementation of this method may ignore
     * the specified timeout. To see the connect timeout set, please
     * call getConnectTimeout().
     *
     * @param timeout an {@code int} that specifies the connect
     *               timeout value in milliseconds
     * @throws IllegalArgumentException if the timeout parameter is negative
     *
     * @see #getConnectTimeout()
     * @see #connect()
     * @since 1.5
     */
    public void setConnectTimeout(int timeout) {
        if (timeout < 0) {
            throw new IllegalArgumentException("timeout can not be negative");
        }
        connectTimeout = timeout;
    }

    /**
     * Returns setting for connect timeout.
     * <p>
     * 0 return implies that the option is disabled
     * (i.e., timeout of infinity).
     *
     * @return an {@code int} that indicates the connect timeout
     *         value in milliseconds
     * @see #setConnectTimeout(int)
     * @see #connect()
     * @since 1.5
     */
    public int getConnectTimeout() {
        return connectTimeout;
    }

    /**
     * Sets the read timeout to a specified timeout, in
     * milliseconds. A non-zero value specifies the timeout when
     * reading from Input stream when a connection is established to a
     * resource. If the timeout expires before there is data available
     * for read, a java.net.SocketTimeoutException is raised. A
     * timeout of zero is interpreted as an infinite timeout.
     *
     *<p> Some non-standard implementation of this method ignores the
     * specified timeout. To see the read timeout set, please call
     * getReadTimeout().
     *
     * @param timeout an {@code int} that specifies the timeout
     * value to be used in milliseconds
     * @throws IllegalArgumentException if the timeout parameter is negative
     *
     * @see #getReadTimeout()
     * @see InputStream#read()
     * @since 1.5
     */
    public void setReadTimeout(int timeout) {
        if (timeout < 0) {
            throw new IllegalArgumentException("timeout can not be negative");
        }
        readTimeout = timeout;
    }

    /**
     * Returns setting for read timeout. 0 return implies that the
     * option is disabled (i.e., timeout of infinity).
     *
     * @return an {@code int} that indicates the read timeout
     *         value in milliseconds
     *
     * @see #setReadTimeout(int)
     * @see InputStream#read()
     * @since 1.5
     */
    public int getReadTimeout() {
        return readTimeout;
    }

    /**
     * Constructs a URL connection to the specified URL. A connection to
     * the object referenced by the URL is not created.
     *
     * @param   url   the specified URL.
     */
    protected URLConnection(URL url) {
        this.url = url;
    }

    /**
     * Returns the value of this {@code URLConnection}'s {@code URL}
     * field.
     *
     * @return  the value of this {@code URLConnection}'s {@code URL}
     *          field.
     * @see     java.net.URLConnection#url
     */
    public URL getURL() {
        return url;
    }

    /**
     * Returns the value of the {@code content-length} header field.
     * <P>
     * <B>Note</B>: {@link #getContentLengthLong() getContentLengthLong()}
     * should be preferred over this method, since it returns a {@code long}
     * instead and is therefore more portable.</P>
     *
     * @return  the content length of the resource that this connection's URL
     *          references, {@code -1} if the content length is not known,
     *          or if the content length is greater than Integer.MAX_VALUE.
     */
    public int getContentLength() {
        long l = getContentLengthLong();
        if (l > Integer.MAX_VALUE)
            return -1;
        return (int) l;
    }

    /**
     * Returns the value of the {@code content-length} header field as a
     * long.
     *
     * @return  the content length of the resource that this connection's URL
     *          references, or {@code -1} if the content length is
     *          not known.
     * @since 7.0
     */
    public long getContentLengthLong() {
        return getHeaderFieldLong("content-length", -1);
    }

    /**
     * Returns the value of the {@code content-type} header field.
     *
     * @return  the content type of the resource that the URL references,
     *          or {@code null} if not known.
     * @see     java.net.URLConnection#getHeaderField(java.lang.String)
     */
    public String getContentType() {
        return getHeaderField("content-type");
    }

    /**
     * Returns the value of the {@code content-encoding} header field.
     *
     * @return  the content encoding of the resource that the URL references,
     *          or {@code null} if not known.
     * @see     java.net.URLConnection#getHeaderField(java.lang.String)
     */
    public String getContentEncoding() {
        return getHeaderField("content-encoding");
    }

    /**
     * Returns the value of the {@code expires} header field.
     *
     * @return  the expiration date of the resource that this URL references,
     *          or 0 if not known. The value is the number of milliseconds since
     *          January 1, 1970 GMT.
     * @see     java.net.URLConnection#getHeaderField(java.lang.String)
     */
    public long getExpiration() {
        return getHeaderFieldDate("expires", 0);
    }

    /**
     * Returns the value of the {@code date} header field.
     *
     * @return  the sending date of the resource that the URL references,
     *          or {@code 0} if not known. The value returned is the
     *          number of milliseconds since January 1, 1970 GMT.
     * @see     java.net.URLConnection#getHeaderField(java.lang.String)
     */
    public long getDate() {
        return getHeaderFieldDate("date", 0);
    }

    /**
     * Returns the value of the {@code last-modified} header field.
     * The result is the number of milliseconds since January 1, 1970 GMT.
     *
     * @return  the date the resource referenced by this
     *          {@code URLConnection} was last modified, or 0 if not known.
     * @see     java.net.URLConnection#getHeaderField(java.lang.String)
     */
    public long getLastModified() {
        return getHeaderFieldDate("last-modified", 0);
    }

    /**
     * Returns the value of the named header field.
     * <p>
     * If called on a connection that sets the same header multiple times
     * with possibly different values, only the last value is returned.
     *
     *
     * @param   name   the name of a header field.
     * @return  the value of the named header field, or {@code null}
     *          if there is no such field in the header.
     */
    public String getHeaderField(String name) {
        return null;
    }

    /**
     * Returns an unmodifiable Map of the header fields.
     * The Map keys are Strings that represent the
     * response-header field names. Each Map value is an
     * unmodifiable List of Strings that represents
     * the corresponding field values.
     *
     * @return a Map of header fields
     * @since 1.4
     */
    public Map<String,List<String>> getHeaderFields() {
        return Collections.emptyMap();
    }

    /**
     * Returns the value of the named field parsed as a number.
     * <p>
     * This form of {@code getHeaderField} exists because some
     * connection types (e.g., {@code http-ng}) have pre-parsed
     * headers. Classes for that connection type can override this method
     * and short-circuit the parsing.
     *
     * @param   name      the name of the header field.
     * @param   Default   the default value.
     * @return  the value of the named field, parsed as an integer. The
     *          {@code Default} value is returned if the field is
     *          missing or malformed.
     */
    public int getHeaderFieldInt(String name, int Default) {
        String value = getHeaderField(name);
        try {
            return Integer.parseInt(value);
        } catch (Exception e) { }
        return Default;
    }

    /**
     * Returns the value of the named field parsed as a number.
     * <p>
     * This form of {@code getHeaderField} exists because some
     * connection types (e.g., {@code http-ng}) have pre-parsed
     * headers. Classes for that connection type can override this method
     * and short-circuit the parsing.
     *
     * @param   name      the name of the header field.
     * @param   Default   the default value.
     * @return  the value of the named field, parsed as a long. The
     *          {@code Default} value is returned if the field is
     *          missing or malformed.
     * @since 7.0
     */
    public long getHeaderFieldLong(String name, long Default) {
        String value = getHeaderField(name);
        try {
            return Long.parseLong(value);
        } catch (Exception e) { }
        return Default;
    }

    /**
     * Returns the value of the named field parsed as date.
     * The result is the number of milliseconds since January 1, 1970 GMT
     * represented by the named field.
     * <p>
     * This form of {@code getHeaderField} exists because some
     * connection types (e.g., {@code http-ng}) have pre-parsed
     * headers. Classes for that connection type can override this method
     * and short-circuit the parsing.
     *
     * @param   name     the name of the header field.
     * @param   Default   a default value.
     * @return  the value of the field, parsed as a date. The value of the
     *          {@code Default} argument is returned if the field is
     *          missing or malformed.
     */
    @SuppressWarnings("deprecation")
    public long getHeaderFieldDate(String name, long Default) {
        String value = getHeaderField(name);
        try {
            return Date.parse(value);
        } catch (Exception e) { }
        return Default;
    }

    /**
     * Returns the key for the {@code n}<sup>th</sup> header field.
     * It returns {@code null} if there are fewer than {@code n+1} fields.
     *
     * @param   n   an index, where {@code n>=0}
     * @return  the key for the {@code n}<sup>th</sup> header field,
     *          or {@code null} if there are fewer than {@code n+1}
     *          fields.
     */
    public String getHeaderFieldKey(int n) {
        return null;
    }

    /**
     * Returns the value for the {@code n}<sup>th</sup> header field.
     * It returns {@code null} if there are fewer than
     * {@code n+1}fields.
     * <p>
     * This method can be used in conjunction with the
     * {@link #getHeaderFieldKey(int) getHeaderFieldKey} method to iterate through all
     * the headers in the message.
     *
     * @param   n   an index, where {@code n>=0}
     * @return  the value of the {@code n}<sup>th</sup> header field
     *          or {@code null} if there are fewer than {@code n+1} fields
     * @see     java.net.URLConnection#getHeaderFieldKey(int)
     */
    public String getHeaderField(int n) {
        return null;
    }

    /**
     * Retrieves the contents of this URL connection.
     * <p>
     * This method first determines the content type of the object by
     * calling the {@code getContentType} method. If this is
     * the first time that the application has seen that specific content
     * type, a content handler for that content type is created:
     * <ol>
     * <li>If the application has set up a content handler factory instance
     *     using the {@code setContentHandlerFactory} method, the
     *     {@code createContentHandler} method of that instance is called
     *     with the content type as an argument; the result is a content
     *     handler for that content type.
     * <li>If no content handler factory has yet been set up, or if the
     *     factory's {@code createContentHandler} method returns
     *     {@code null}, then the application loads the class named:
     *     <blockquote><pre>
     *         sun.net.www.content.&lt;<i>contentType</i>&gt;
     *     </pre></blockquote>
     *     where &lt;<i>contentType</i>&gt; is formed by taking the
     *     content-type string, replacing all slash characters with a
     *     {@code period} ('.'), and all other non-alphanumeric characters
     *     with the underscore character '{@code _}'. The alphanumeric
     *     characters are specifically the 26 uppercase ASCII letters
     *     '{@code A}' through '{@code Z}', the 26 lowercase ASCII
     *     letters '{@code a}' through '{@code z}', and the 10 ASCII
     *     digits '{@code 0}' through '{@code 9}'. If the specified
     *     class does not exist, or is not a subclass of
     *     {@code ContentHandler}, then an
     *     {@code UnknownServiceException} is thrown.
     * </ol>
     *
     * @return     the object fetched. The {@code instanceof} operator
     *               should be used to determine the specific kind of object
     *               returned.
     * @exception  IOException              if an I/O error occurs while
     *               getting the content.
     * @exception  UnknownServiceException  if the protocol does not support
     *               the content type.
     * @see        java.net.ContentHandlerFactory#createContentHandler(java.lang.String)
     * @see        java.net.URLConnection#getContentType()
     * @see        java.net.URLConnection#setContentHandlerFactory(java.net.ContentHandlerFactory)
     */
    public Object getContent() throws IOException {
        // Must call getInputStream before GetHeaderField gets called
        // so that FileNotFoundException has a chance to be thrown up
        // from here without being caught.
        getInputStream();
        return getContentHandler().getContent(this);
    }

    /**
     * Retrieves the contents of this URL connection.
     *
     * @param classes the {@code Class} array
     * indicating the requested types
     * @return     the object fetched that is the first match of the type
     *               specified in the classes array. null if none of
     *               the requested types are supported.
     *               The {@code instanceof} operator should be used to
     *               determine the specific kind of object returned.
     * @exception  IOException              if an I/O error occurs while
     *               getting the content.
     * @exception  UnknownServiceException  if the protocol does not support
     *               the content type.
     * @see        java.net.URLConnection#getContent()
     * @see        java.net.ContentHandlerFactory#createContentHandler(java.lang.String)
     * @see        java.net.URLConnection#getContent(java.lang.Class[])
     * @see        java.net.URLConnection#setContentHandlerFactory(java.net.ContentHandlerFactory)
     * @since 1.3
     */
    public Object getContent(Class[] classes) throws IOException {
        // Must call getInputStream before GetHeaderField gets called
        // so that FileNotFoundException has a chance to be thrown up
        // from here without being caught.
        getInputStream();
        return getContentHandler().getContent(this, classes);
    }

    /**
     * Returns a permission object representing the permission
     * necessary to make the connection represented by this
     * object. This method returns null if no permission is
     * required to make the connection. By default, this method
     * returns {@code java.security.AllPermission}. Subclasses
     * should override this method and return the permission
     * that best represents the permission required to make a
     * a connection to the URL. For example, a {@code URLConnection}
     * representing a {@code file:} URL would return a
     * {@code java.io.FilePermission} object.
     *
     * <p>The permission returned may dependent upon the state of the
     * connection. For example, the permission before connecting may be
     * different from that after connecting. For example, an HTTP
     * sever, say foo.com, may redirect the connection to a different
     * host, say bar.com. Before connecting the permission returned by
     * the connection will represent the permission needed to connect
     * to foo.com, while the permission returned after connecting will
     * be to bar.com.
     *
     * <p>Permissions are generally used for two purposes: to protect
     * caches of objects obtained through URLConnections, and to check
     * the right of a recipient to learn about a particular URL. In
     * the first case, the permission should be obtained
     * <em>after</em> the object has been obtained. For example, in an
     * HTTP connection, this will represent the permission to connect
     * to the host from which the data was ultimately fetched. In the
     * second case, the permission should be obtained and tested
     * <em>before</em> connecting.
     *
     * @return the permission object representing the permission
     * necessary to make the connection represented by this
     * URLConnection.
     *
     * @exception IOException if the computation of the permission
     * requires network or file I/O and an exception occurs while
     * computing it.
     */
    public Permission getPermission() throws IOException {
        return SecurityConstants.ALL_PERMISSION;
    }

    /**
     * Returns an input stream that reads from this open connection.
     *
     * A SocketTimeoutException can be thrown when reading from the
     * returned input stream if the read timeout expires before data
     * is available for read.
     *
     * @return     an input stream that reads from this open connection.
     * @exception  IOException              if an I/O error occurs while
     *               creating the input stream.
     * @exception  UnknownServiceException  if the protocol does not support
     *               input.
     * @see #setReadTimeout(int)
     * @see #getReadTimeout()
     */
    public InputStream getInputStream() throws IOException {
        throw new UnknownServiceException("protocol doesn't support input");
    }

    /**
     * Returns an output stream that writes to this connection.
     *
     * @return     an output stream that writes to this connection.
     * @exception  IOException              if an I/O error occurs while
     *               creating the output stream.
     * @exception  UnknownServiceException  if the protocol does not support
     *               output.
     */
    public OutputStream getOutputStream() throws IOException {
        throw new UnknownServiceException("protocol doesn't support output");
    }

    /**
     * Returns a {@code String} representation of this URL connection.
     *
     * @return  a string representation of this {@code URLConnection}.
     */
    public String toString() {
        return this.getClass().getName() + ":" + url;
    }

    /**
     * Sets the value of the {@code doInput} field for this
     * {@code URLConnection} to the specified value.
     * <p>
     * A URL connection can be used for input and/or output.  Set the DoInput
     * flag to true if you intend to use the URL connection for input,
     * false if not.  The default is true.
     *
     * @param   doinput   the new value.
     * @throws IllegalStateException if already connected
     * @see     java.net.URLConnection#doInput
     * @see #getDoInput()
     */
    public void setDoInput(boolean doinput) {
        if (connected)
            throw new IllegalStateException("Already connected");
        doInput = doinput;
    }

    /**
     * Returns the value of this {@code URLConnection}'s
     * {@code doInput} flag.
     *
     * @return  the value of this {@code URLConnection}'s
     *          {@code doInput} flag.
     * @see     #setDoInput(boolean)
     */
    public boolean getDoInput() {
        return doInput;
    }

    /**
     * Sets the value of the {@code doOutput} field for this
     * {@code URLConnection} to the specified value.
     * <p>
     * A URL connection can be used for input and/or output.  Set the DoOutput
     * flag to true if you intend to use the URL connection for output,
     * false if not.  The default is false.
     *
     * @param   dooutput   the new value.
     * @throws IllegalStateException if already connected
     * @see #getDoOutput()
     */
    public void setDoOutput(boolean dooutput) {
        if (connected)
            throw new IllegalStateException("Already connected");
        doOutput = dooutput;
    }

    /**
     * Returns the value of this {@code URLConnection}'s
     * {@code doOutput} flag.
     *
     * @return  the value of this {@code URLConnection}'s
     *          {@code doOutput} flag.
     * @see     #setDoOutput(boolean)
     */
    public boolean getDoOutput() {
        return doOutput;
    }

    /**
     * Set the value of the {@code allowUserInteraction} field of
     * this {@code URLConnection}.
     *
     * @param   allowuserinteraction   the new value.
     * @throws IllegalStateException if already connected
     * @see     #getAllowUserInteraction()
     */
    public void setAllowUserInteraction(boolean allowuserinteraction) {
        if (connected)
            throw new IllegalStateException("Already connected");
        allowUserInteraction = allowuserinteraction;
    }

    /**
     * Returns the value of the {@code allowUserInteraction} field for
     * this object.
     *
     * @return  the value of the {@code allowUserInteraction} field for
     *          this object.
     * @see     #setAllowUserInteraction(boolean)
     */
    public boolean getAllowUserInteraction() {
        return allowUserInteraction;
    }

    /**
     * Sets the default value of the
     * {@code allowUserInteraction} field for all future
     * {@code URLConnection} objects to the specified value.
     *
     * @param   defaultallowuserinteraction   the new value.
     * @see     #getDefaultAllowUserInteraction()
     */
    public static void setDefaultAllowUserInteraction(boolean defaultallowuserinteraction) {
        defaultAllowUserInteraction = defaultallowuserinteraction;
    }

    /**
     * Returns the default value of the {@code allowUserInteraction}
     * field.
     * <p>
     * Ths default is "sticky", being a part of the static state of all
     * URLConnections.  This flag applies to the next, and all following
     * URLConnections that are created.
     *
     * @return  the default value of the {@code allowUserInteraction}
     *          field.
     * @see     #setDefaultAllowUserInteraction(boolean)
     */
    public static boolean getDefaultAllowUserInteraction() {
        return defaultAllowUserInteraction;
    }

    /**
     * Sets the value of the {@code useCaches} field of this
     * {@code URLConnection} to the specified value.
     * <p>
     * Some protocols do caching of documents.  Occasionally, it is important
     * to be able to "tunnel through" and ignore the caches (e.g., the
     * "reload" button in a browser).  If the UseCaches flag on a connection
     * is true, the connection is allowed to use whatever caches it can.
     *  If false, caches are to be ignored.
     *  The default value comes from DefaultUseCaches, which defaults to
     * true.
     *
     * @param usecaches a {@code boolean} indicating whether
     * or not to allow caching
     * @throws IllegalStateException if already connected
     * @see #getUseCaches()
     */
    public void setUseCaches(boolean usecaches) {
        if (connected)
            throw new IllegalStateException("Already connected");
        useCaches = usecaches;
    }

    /**
     * Returns the value of this {@code URLConnection}'s
     * {@code useCaches} field.
     *
     * @return  the value of this {@code URLConnection}'s
     *          {@code useCaches} field.
     * @see #setUseCaches(boolean)
     */