Class HttpURLConnection
- java.lang.Object
-
- java.net.URLConnection
-
- java.net.HttpURLConnection
- Direct Known Subclasses:
HttpsURLConnection
public abstract class HttpURLConnection extends URLConnection
A URLConnection with support for HTTP-specific features. See the spec for details.
Each HttpURLConnection instance is used to make a single request but the underlying network connection to the HTTP server may be transparently shared by other instances. Calling the close() methods on the InputStream or OutputStream of an HttpURLConnection after a request may free network resources associated with this instance but has no effect on any shared persistent connection. Calling the disconnect() method may close the underlying socket if a persistent connection is otherwise idle at that time.
The HTTP protocol handler has a few settings that can be accessed through System Properties. This covers Proxy settings as well as various other settings.
Security permissions
If a security manager is installed, and if a method is called which results in an attempt to open a connection, the caller must possess either:
- a "connect"
SocketPermission
to the host/port combination of the destination URL or - a
URLPermission
that permits this request.
If automatic redirection is enabled, and this request is redirected to another destination, then the caller must also have permission to connect to the redirected host/URL.
- Since:
- 1.1
- See Also:
disconnect()
Field Summary
Modifier and Type | Field | Description |
---|---|---|
protected int | chunkLength | The chunk-length when using chunked encoding streaming mode for output. |
protected int | fixedContentLength | The fixed content-length when using fixed-length streaming mode. |
protected long | fixedContentLengthLong | The fixed content-length when using fixed-length streaming mode. |
static int | HTTP_ACCEPTED | HTTP Status-Code 202: Accepted. |
static int | HTTP_BAD_GATEWAY | HTTP Status-Code 502: Bad Gateway. |
static int | HTTP_BAD_METHOD | HTTP Status-Code 405: Method Not Allowed. |
static int | HTTP_BAD_REQUEST | HTTP Status-Code 400: Bad Request. |
static int | HTTP_CLIENT_TIMEOUT | HTTP Status-Code 408: Request Time-Out. |
static int | HTTP_CONFLICT | HTTP Status-Code 409: Conflict. |
static int | HTTP_CREATED | HTTP Status-Code 201: Created. |
static int | HTTP_ENTITY_TOO_LARGE | HTTP Status-Code 413: Request Entity Too Large. |
static int | HTTP_FORBIDDEN | HTTP Status-Code 403: Forbidden. |
static int | HTTP_GATEWAY_TIMEOUT | HTTP Status-Code 504: Gateway Timeout. |
static int | HTTP_GONE | HTTP Status-Code 410: Gone. |
static int | HTTP_INTERNAL_ERROR | HTTP Status-Code 500: Internal Server Error. |
static int | HTTP_LENGTH_REQUIRED | HTTP Status-Code 411: Length Required. |
static int | HTTP_MOVED_PERM | HTTP Status-Code 301: Moved Permanently. |
static int | HTTP_MOVED_TEMP | HTTP Status-Code 302: Temporary Redirect. |
static int | HTTP_MULT_CHOICE | HTTP Status-Code 300: Multiple Choices. |
static int | HTTP_NO_CONTENT | HTTP Status-Code 204: No Content. |
static int | HTTP_NOT_ACCEPTABLE | HTTP Status-Code 406: Not Acceptable. |
static int | HTTP_NOT_AUTHORITATIVE | HTTP Status-Code 203: Non-Authoritative Information. |
static int | HTTP_NOT_FOUND | HTTP Status-Code 404: Not Found. |
static int | HTTP_NOT_IMPLEMENTED | HTTP Status-Code 501: Not Implemented. |
static int | HTTP_NOT_MODIFIED | HTTP Status-Code 304: Not Modified. |
static int | HTTP_OK | HTTP Status-Code 200: OK. |
static int | HTTP_PARTIAL | HTTP Status-Code 206: Partial Content. |
static int | HTTP_PAYMENT_REQUIRED | HTTP Status-Code 402: Payment Required. |
static int | HTTP_PRECON_FAILED | HTTP Status-Code 412: Precondition Failed. |
static int | HTTP_PROXY_AUTH | HTTP Status-Code 407: Proxy Authentication Required. |
static int | HTTP_REQ_TOO_LONG | HTTP Status-Code 414: Request-URI Too Large. |
static int | HTTP_RESET | HTTP Status-Code 205: Reset Content. |
static int | HTTP_SEE_OTHER | HTTP Status-Code 303: See Other. |
static int | HTTP_SERVER_ERROR | Deprecated. it is misplaced and shouldn't have existed. |
static int | HTTP_UNAUTHORIZED | HTTP Status-Code 401: Unauthorized. |
static int | HTTP_UNAVAILABLE | HTTP Status-Code 503: Service Unavailable. |
static int | HTTP_UNSUPPORTED_TYPE | HTTP Status-Code 415: Unsupported Media Type. |
static int | HTTP_USE_PROXY | HTTP Status-Code 305: Use Proxy. |
static int | HTTP_VERSION | HTTP Status-Code 505: HTTP Version Not Supported. |
protected boolean | instanceFollowRedirects | If |
protected String | method | The HTTP method (GET,POST,PUT,etc.). |
protected int | responseCode | An |
protected String | responseMessage | The HTTP response message. |
Fields declared in class java.net.URLConnection
allowUserInteraction, connected, doInput, doOutput, ifModifiedSince, url, useCaches
Constructor Summary
Modifier | Constructor | Description |
---|---|---|
protected | HttpURLConnection(URL u) | Constructor for the HttpURLConnection. |
Method Summary
Modifier and Type | Method | Description |
---|---|---|
abstract void | disconnect() | Indicates that other requests to the server are unlikely in the near future. |
InputStream | getErrorStream() | Returns the error stream if the connection failed but the server sent useful data nonetheless. |
static boolean | getFollowRedirects() | Returns a |
String | getHeaderField(int n) | Returns the value for the |
String | getHeaderFieldKey(int n) | Returns the key for the |
boolean | getInstanceFollowRedirects() | Returns the value of this |
Permission | getPermission() | Returns a |
String | getRequestMethod() | Get the request method. |
int | getResponseCode() | Gets the status code from an HTTP response message. |
String | getResponseMessage() | Gets the HTTP response message, if any, returned along with the response code from a server. |
void | setAuthenticator(Authenticator auth) | Supplies an |
void | setChunkedStreamingMode(int chunklen) | This method is used to enable streaming of a HTTP request body without internal buffering, when the content length is not known in advance. |
void | setFixedLengthStreamingMode(int contentLength) | This method is used to enable streaming of a HTTP request body without internal buffering, when the content length is known in advance. |
void | setFixedLengthStreamingMode(long contentLength) | This method is used to enable streaming of a HTTP request body without internal buffering, when the content length is known in advance. |
static void | setFollowRedirects(boolean set) | Sets whether HTTP redirects (requests with response code 3xx) should be automatically followed by this class. |
void | setInstanceFollowRedirects(boolean followRedirects) | Sets whether HTTP redirects (requests with response code 3xx) should be automatically followed by this |
void | setRequestMethod(String method) | Set the method for the URL request, one of: GET POST HEAD OPTIONS PUT DELETE TRACE are legal, subject to protocol restrictions. |
abstract boolean | usingProxy() | Indicates if the connection is going through a proxy. |
Methods declared in class java.net.URLConnection
addRequestProperty, connect, getAllowUserInteraction, getConnectTimeout, getContent, getContent, getContentEncoding, getContentLength, getContentLengthLong, getContentType, getDate, getDefaultAllowUserInteraction, getDefaultRequestProperty, getDefaultUseCaches, getDefaultUseCaches, getDoInput, getDoOutput, getExpiration, getFileNameMap, getHeaderField, getHeaderFieldDate, getHeaderFieldInt, getHeaderFieldLong, getHeaderFields, getIfModifiedSince, getInputStream, getLastModified, getOutputStream, getReadTimeout, getRequestProperties, getRequestProperty, getURL, getUseCaches, guessContentTypeFromName, guessContentTypeFromStream, setAllowUserInteraction, setConnectTimeout, setContentHandlerFactory, setDefaultAllowUserInteraction, setDefaultRequestProperty, setDefaultUseCaches, setDefaultUseCaches, setDoInput, setDoOutput, setFileNameMap, setIfModifiedSince, setReadTimeout, setRequestProperty, setUseCaches, toString
Methods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
Field Detail
method
protected String method
The HTTP method (GET,POST,PUT,etc.).
chunkLength
protected int chunkLength
The chunk-length when using chunked encoding streaming mode for output. A value of -1
means chunked encoding is disabled for output.
- Since:
- 1.5
fixedContentLength
protected int fixedContentLength
The fixed content-length when using fixed-length streaming mode. A value of -1
means fixed-length streaming mode is disabled for output.
NOTE: fixedContentLengthLong
is recommended instead of this field, as it allows larger content lengths to be set.
- Since:
- 1.5
fixedContentLengthLong
protected long fixedContentLengthLong
The fixed content-length when using fixed-length streaming mode. A value of -1
means fixed-length streaming mode is disabled for output.
- Since:
- 1.7
responseCode
protected int responseCode
An int
representing the three digit HTTP Status-Code.
- 1xx: Informational
- 2xx: Success
- 3xx: Redirection
- 4xx: Client Error
- 5xx: Server Error
responseMessage
protected String responseMessage
The HTTP response message.
instanceFollowRedirects
protected boolean instanceFollowRedirects
If true
, the protocol will automatically follow redirects. If false
, the protocol will not automatically follow redirects.
This field is set by the setInstanceFollowRedirects
method. Its value is returned by the getInstanceFollowRedirects
method.
Its default value is based on the value of the static followRedirects at HttpURLConnection construction time.
- See Also:
-
setInstanceFollowRedirects(boolean)
,getInstanceFollowRedirects()
,setFollowRedirects(boolean)
HTTP_OK
public static final int HTTP_OK
HTTP Status-Code 200: OK.
- See Also:
- Constant Field Values
HTTP_CREATED
public static final int HTTP_CREATED
HTTP Status-Code 201: Created.
- See Also:
- Constant Field Values
HTTP_ACCEPTED
public static final int HTTP_ACCEPTED
HTTP Status-Code 202: Accepted.
- See Also:
- Constant Field Values
HTTP_NOT_AUTHORITATIVE
public static final int HTTP_NOT_AUTHORITATIVE
HTTP Status-Code 203: Non-Authoritative Information.
- See Also:
- Constant Field Values
HTTP_NO_CONTENT
public static final int HTTP_NO_CONTENT
HTTP Status-Code 204: No Content.
- See Also:
- Constant Field Values
HTTP_RESET
public static final int HTTP_RESET
HTTP Status-Code 205: Reset Content.
- See Also:
- Constant Field Values
HTTP_PARTIAL
public static final int HTTP_PARTIAL
HTTP Status-Code 206: Partial Content.
- See Also:
- Constant Field Values
HTTP_MULT_CHOICE
public static final int HTTP_MULT_CHOICE
HTTP Status-Code 300: Multiple Choices.
- See Also:
- Constant Field Values
HTTP_MOVED_PERM
public static final int HTTP_MOVED_PERM
HTTP Status-Code 301: Moved Permanently.
- See Also:
- Constant Field Values
HTTP_MOVED_TEMP
public static final int HTTP_MOVED_TEMP
HTTP Status-Code 302: Temporary Redirect.
- See Also:
- Constant Field Values
HTTP_SEE_OTHER
public static final int HTTP_SEE_OTHER
HTTP Status-Code 303: See Other.
- See Also:
- Constant Field Values
HTTP_NOT_MODIFIED
public static final int HTTP_NOT_MODIFIED
HTTP Status-Code 304: Not Modified.
- See Also:
- Constant Field Values
HTTP_USE_PROXY
public static final int HTTP_USE_PROXY
HTTP Status-Code 305: Use Proxy.
- See Also:
- Constant Field Values
HTTP_BAD_REQUEST
public static final int HTTP_BAD_REQUEST
HTTP Status-Code 400: Bad Request.
- See Also:
- Constant Field Values
HTTP_UNAUTHORIZED
public static final int HTTP_UNAUTHORIZED
HTTP Status-Code 401: Unauthorized.
- See Also:
- Constant Field Values
HTTP_PAYMENT_REQUIRED
public static final int HTTP_PAYMENT_REQUIRED
HTTP Status-Code 402: Payment Required.
- See Also:
- Constant Field Values
HTTP_FORBIDDEN
public static final int HTTP_FORBIDDEN
HTTP Status-Code 403: Forbidden.
- See Also:
- Constant Field Values
HTTP_NOT_FOUND
public static final int HTTP_NOT_FOUND
HTTP Status-Code 404: Not Found.
- See Also:
- Constant Field Values
HTTP_BAD_METHOD
public static final int HTTP_BAD_METHOD
HTTP Status-Code 405: Method Not Allowed.
- See Also:
- Constant Field Values
HTTP_NOT_ACCEPTABLE
public static final int HTTP_NOT_ACCEPTABLE
HTTP Status-Code 406: Not Acceptable.
- See Also:
- Constant Field Values
HTTP_PROXY_AUTH
public static final int HTTP_PROXY_AUTH
HTTP Status-Code 407: Proxy Authentication Required.
- See Also:
- Constant Field Values
HTTP_CLIENT_TIMEOUT
public static final int HTTP_CLIENT_TIMEOUT
HTTP Status-Code 408: Request Time-Out.
- See Also:
- Constant Field Values
HTTP_CONFLICT
public static final int HTTP_CONFLICT
HTTP Status-Code 409: Conflict.
- See Also:
- Constant Field Values
HTTP_GONE
public static final int HTTP_GONE
HTTP Status-Code 410: Gone.
- See Also:
- Constant Field Values
HTTP_LENGTH_REQUIRED
public static final int HTTP_LENGTH_REQUIRED
HTTP Status-Code 411: Length Required.
- See Also:
- Constant Field Values
HTTP_PRECON_FAILED
public static final int HTTP_PRECON_FAILED
HTTP Status-Code 412: Precondition Failed.
- See Also:
- Constant Field Values
HTTP_ENTITY_TOO_LARGE
public static final int HTTP_ENTITY_TOO_LARGE
HTTP Status-Code 413: Request Entity Too Large.
- See Also:
- Constant Field Values
HTTP_REQ_TOO_LONG
public static final int HTTP_REQ_TOO_LONG
HTTP Status-Code 414: Request-URI Too Large.
- See Also:
- Constant Field Values
HTTP_UNSUPPORTED_TYPE
public static final int HTTP_UNSUPPORTED_TYPE
HTTP Status-Code 415: Unsupported Media Type.
- See Also:
- Constant Field Values
HTTP_SERVER_ERROR
@Deprecated public static final int HTTP_SERVER_ERROR
HTTP Status-Code 500: Internal Server Error.
- See Also:
- Constant Field Values
HTTP_INTERNAL_ERROR
public static final int HTTP_INTERNAL_ERROR
HTTP Status-Code 500: Internal Server Error.
- See Also:
- Constant Field Values
HTTP_NOT_IMPLEMENTED
public static final int HTTP_NOT_IMPLEMENTED
HTTP Status-Code 501: Not Implemented.
- See Also:
- Constant Field Values
HTTP_BAD_GATEWAY
public static final int HTTP_BAD_GATEWAY
HTTP Status-Code 502: Bad Gateway.
- See Also:
- Constant Field Values
HTTP_UNAVAILABLE
public static final int HTTP_UNAVAILABLE
HTTP Status-Code 503: Service Unavailable.
- See Also:
- Constant Field Values
HTTP_GATEWAY_TIMEOUT
public static final int HTTP_GATEWAY_TIMEOUT
HTTP Status-Code 504: Gateway Timeout.
- See Also:
- Constant Field Values
HTTP_VERSION
public static final int HTTP_VERSION
HTTP Status-Code 505: HTTP Version Not Supported.
- See Also:
- Constant Field Values
Constructor Detail
HttpURLConnection
protected HttpURLConnection(URL u)
Constructor for the HttpURLConnection.
- Parameters:
-
u
- the URL
Method Detail
setAuthenticator
public void setAuthenticator(Authenticator auth)
Supplies an Authenticator
to be used when authentication is requested through the HTTP protocol for this HttpURLConnection
. If no authenticator is supplied, the default authenticator will be used.
- Implementation Requirements:
- The default behavior of this method is to unconditionally throw
UnsupportedOperationException
. Concrete implementations ofHttpURLConnection
which support supplying anAuthenticator
for a specificHttpURLConnection
instance should override this method to implement a different behavior. - Implementation Note:
- Depending on authentication schemes, an implementation may or may not need to use the provided authenticator to obtain a password. For instance, an implementation that relies on third-party security libraries may still invoke the default authenticator if these libraries are configured to do so. Likewise, an implementation that supports transparent NTLM authentication may let the system attempt to connect using the system user credentials first, before invoking the provided authenticator.
However, if an authenticator is specifically provided, then the underlying connection may only be reused forHttpURLConnection
instances which share the sameAuthenticator
instance, and authentication information, if cached, may only be reused for anHttpURLConnection
sharing that sameAuthenticator
. - Parameters:
-
auth
- TheAuthenticator
that should be used by thisHttpURLConnection
. - Throws:
-
UnsupportedOperationException
- if setting an Authenticator is not supported by the underlying implementation. -
IllegalStateException
- if URLConnection is already connected. -
NullPointerException
- if the suppliedauth
isnull
. - Since:
- 9
getHeaderFieldKey
public String getHeaderFieldKey(int n)
Returns the key for the n
th header field. Some implementations may treat the 0
th header field as special, i.e. as the status line returned by the HTTP server. In this case, getHeaderField(0)
returns the status line, but getHeaderFieldKey(0)
returns null.
- Overrides:
-
getHeaderFieldKey
in classURLConnection
- Parameters:
-
n
- an index, wheren >=0
. - Returns:
- the key for the
n
th header field, ornull
if the key does not exist.
setFixedLengthStreamingMode
public void setFixedLengthStreamingMode(int contentLength)
This method is used to enable streaming of a HTTP request body without internal buffering, when the content length is known in advance.
An exception will be thrown if the application attempts to write more data than the indicated content-length, or if the application closes the OutputStream before writing the indicated amount.
When output streaming is enabled, authentication and redirection cannot be handled automatically. A HttpRetryException will be thrown when reading the response if authentication or redirection are required. This exception can be queried for the details of the error.
This method must be called before the URLConnection is connected.
NOTE: setFixedLengthStreamingMode(long)
is recommended instead of this method as it allows larger content lengths to be set.
- Parameters:
-
contentLength
- The number of bytes which will be written to the OutputStream. - Throws:
-
IllegalStateException
- if URLConnection is already connected or if a different streaming mode is already enabled. -
IllegalArgumentException
- if a content length less than zero is specified. - Since:
- 1.5
- See Also:
setChunkedStreamingMode(int)
setFixedLengthStreamingMode
public void setFixedLengthStreamingMode(long contentLength)
This method is used to enable streaming of a HTTP request body without internal buffering, when the content length is known in advance.
An exception will be thrown if the application attempts to write more data than the indicated content-length, or if the application closes the OutputStream before writing the indicated amount.
When output streaming is enabled, authentication and redirection cannot be handled automatically. A HttpRetryException will be thrown when reading the response if authentication or redirection are required. This exception can be queried for the details of the error.
This method must be called before the URLConnection is connected.
The content length set by invoking this method takes precedence over any value set by setFixedLengthStreamingMode(int)
.
- Parameters:
-
contentLength
- The number of bytes which will be written to the OutputStream. - Throws:
-
IllegalStateException
- if URLConnection is already connected or if a different streaming mode is already enabled. -
IllegalArgumentException
- if a content length less than zero is specified. - Since:
- 1.7
setChunkedStreamingMode
public void setChunkedStreamingMode(int chunklen)
This method is used to enable streaming of a HTTP request body without internal buffering, when the content length is not known in advance. In this mode, chunked transfer encoding is used to send the request body. Note, not all HTTP servers support this mode.
When output streaming is enabled, authentication and redirection cannot be handled automatically. A HttpRetryException will be thrown when reading the response if authentication or redirection are required. This exception can be queried for the details of the error.
This method must be called before the URLConnection is connected.
- Parameters:
-
chunklen
- The number of bytes to write in each chunk. If chunklen is less than or equal to zero, a default value will be used. - Throws:
-
IllegalStateException
- if URLConnection is already connected or if a different streaming mode is already enabled. - Since:
- 1.5
- See Also:
setFixedLengthStreamingMode(int)
getHeaderField
public String getHeaderField(int n)
Returns the value for the n
th header field. Some implementations may treat the 0
th header field as special, i.e. as the status line returned by the HTTP server.
This method can be used in conjunction with the getHeaderFieldKey
method to iterate through all the headers in the message.
- Overrides:
-
getHeaderField
in classURLConnection
- Parameters:
-
n
- an index, wheren>=0
. - Returns:
- the value of the
n
th header field, ornull
if the value does not exist. - See Also:
getHeaderFieldKey(int)
setFollowRedirects
public static void setFollowRedirects(boolean set)
Sets whether HTTP redirects (requests with response code 3xx) should be automatically followed by this class. True by default. Applets cannot change this variable.
If there is a security manager, this method first calls the security manager's checkSetFactory
method to ensure the operation is allowed. This could result in a SecurityException.
- Parameters:
-
set
- aboolean
indicating whether or not to follow HTTP redirects. - Throws:
-
SecurityException
- if a security manager exists and itscheckSetFactory
method doesn't allow the operation. - See Also:
-
SecurityManager.checkSetFactory()
,getFollowRedirects()
getFollowRedirects
public static boolean getFollowRedirects()
Returns a boolean
indicating whether or not HTTP redirects (3xx) should be automatically followed.
- Returns:
-
true
if HTTP redirects should be automatically followed,false
if not. - See Also:
setFollowRedirects(boolean)
setInstanceFollowRedirects
public void setInstanceFollowRedirects(boolean followRedirects)
Sets whether HTTP redirects (requests with response code 3xx) should be automatically followed by this HttpURLConnection
instance.
The default value comes from followRedirects, which defaults to true.
- Parameters:
-
followRedirects
- aboolean
indicating whether or not to follow HTTP redirects. - Since:
- 1.3
- See Also:
-
instanceFollowRedirects
,getInstanceFollowRedirects()
getInstanceFollowRedirects
public boolean getInstanceFollowRedirects()
Returns the value of this HttpURLConnection
's instanceFollowRedirects
field.
- Returns:
- the value of this
HttpURLConnection
'sinstanceFollowRedirects
field. - Since:
- 1.3
- See Also:
-
instanceFollowRedirects
,setInstanceFollowRedirects(boolean)
setRequestMethod
public void setRequestMethod(String method) throws ProtocolException
Set the method for the URL request, one of:
- GET
- POST
- HEAD
- OPTIONS
- PUT
- DELETE
- TRACE
- Parameters:
-
method
- the HTTP method - Throws:
-
ProtocolException
- if the method cannot be reset or if the requested method isn't valid for HTTP. -
SecurityException
- if a security manager is set and the method is "TRACE", but the "allowHttpTrace" NetPermission is not granted. - See Also:
getRequestMethod()
getRequestMethod
public String getRequestMethod()
Get the request method.
- Returns:
- the HTTP request method
- See Also:
setRequestMethod(java.lang.String)
getResponseCode
public int getResponseCode() throws IOException
Gets the status code from an HTTP response message. For example, in the case of the following status lines:
HTTP/1.0 200 OK HTTP/1.0 401 UnauthorizedIt will return 200 and 401 respectively. Returns -1 if no code can be discerned from the response (i.e., the response is not valid HTTP).
- Returns:
- the HTTP Status-Code, or -1
- Throws:
-
IOException
- if an error occurred connecting to the server.
getResponseMessage
public String getResponseMessage() throws IOException
Gets the HTTP response message, if any, returned along with the response code from a server. From responses like:
HTTP/1.0 200 OK HTTP/1.0 404 Not FoundExtracts the Strings "OK" and "Not Found" respectively. Returns null if none could be discerned from the responses (the result was not valid HTTP).
- Returns:
- the HTTP response message, or
null
- Throws:
-
IOException
- if an error occurred connecting to the server.
disconnect
public abstract void disconnect()
Indicates that other requests to the server are unlikely in the near future. Calling disconnect() should not imply that this HttpURLConnection instance can be reused for other requests.
usingProxy
public abstract boolean usingProxy()
Indicates if the connection is going through a proxy.
- Returns:
- a boolean indicating if the connection is using a proxy.
getPermission
public Permission getPermission() throws IOException
Returns a SocketPermission
object representing the permission necessary to connect to the destination host and port.
- Overrides:
-
getPermission
in classURLConnection
- Returns:
- a
SocketPermission
object representing the permission necessary to connect to the destination host and port. - Throws:
-
IOException
- if an error occurs while computing the permission.
getErrorStream
public InputStream getErrorStream()
Returns the error stream if the connection failed but the server sent useful data nonetheless. The typical example is when an HTTP server responds with a 404, which will cause a FileNotFoundException to be thrown in connect, but the server sent an HTML help page with suggestions as to what to do.
This method will not cause a connection to be initiated. If the connection was not connected, or if the server did not have an error while connecting or if the server had an error but no error data was sent, this method will return null. This is the default.
- Returns:
- an error stream if any, null if there have been no errors, the connection is not connected or the server sent no useful data.