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 final int`	`HTTP_ACCEPTED`	HTTP Status-Code 202: Accepted.
`static final int`	`HTTP_BAD_GATEWAY`	HTTP Status-Code 502: Bad Gateway.
`static final int`	`HTTP_BAD_METHOD`	HTTP Status-Code 405: Method Not Allowed.
`static final int`	`HTTP_BAD_REQUEST`	HTTP Status-Code 400: Bad Request.
`static final int`	`HTTP_CLIENT_TIMEOUT`	HTTP Status-Code 408: Request Time-Out.
`static final int`	`HTTP_CONFLICT`	HTTP Status-Code 409: Conflict.
`static final int`	`HTTP_CREATED`	HTTP Status-Code 201: Created.
`static final int`	`HTTP_ENTITY_TOO_LARGE`	HTTP Status-Code 413: Request Entity Too Large.
`static final int`	`HTTP_FORBIDDEN`	HTTP Status-Code 403: Forbidden.
`static final int`	`HTTP_GATEWAY_TIMEOUT`	HTTP Status-Code 504: Gateway Timeout.
`static final int`	`HTTP_GONE`	HTTP Status-Code 410: Gone.
`static final int`	`HTTP_INTERNAL_ERROR`	HTTP Status-Code 500: Internal Server Error.
`static final int`	`HTTP_LENGTH_REQUIRED`	HTTP Status-Code 411: Length Required.
`static final int`	`HTTP_MOVED_PERM`	HTTP Status-Code 301: Moved Permanently.
`static final int`	`HTTP_MOVED_TEMP`	HTTP Status-Code 302: Temporary Redirect.
`static final int`	`HTTP_MULT_CHOICE`	HTTP Status-Code 300: Multiple Choices.
`static final int`	`HTTP_NO_CONTENT`	HTTP Status-Code 204: No Content.
`static final int`	`HTTP_NOT_ACCEPTABLE`	HTTP Status-Code 406: Not Acceptable.
`static final int`	`HTTP_NOT_AUTHORITATIVE`	HTTP Status-Code 203: Non-Authoritative Information.
`static final int`	`HTTP_NOT_FOUND`	HTTP Status-Code 404: Not Found.
`static final int`	`HTTP_NOT_IMPLEMENTED`	HTTP Status-Code 501: Not Implemented.
`static final int`	`HTTP_NOT_MODIFIED`	HTTP Status-Code 304: Not Modified.
`static final int`	`HTTP_OK`	HTTP Status-Code 200: OK.
`static final int`	`HTTP_PARTIAL`	HTTP Status-Code 206: Partial Content.
`static final int`	`HTTP_PAYMENT_REQUIRED`	HTTP Status-Code 402: Payment Required.
`static final int`	`HTTP_PRECON_FAILED`	HTTP Status-Code 412: Precondition Failed.
`static final int`	`HTTP_PROXY_AUTH`	HTTP Status-Code 407: Proxy Authentication Required.
`static final int`	`HTTP_REQ_TOO_LONG`	HTTP Status-Code 414: Request-URI Too Large.
`static final int`	`HTTP_RESET`	HTTP Status-Code 205: Reset Content.
`static final int`	`HTTP_SEE_OTHER`	HTTP Status-Code 303: See Other.
`static final int`	`HTTP_SERVER_ERROR`	Deprecated. it is misplaced and shouldn't have existed.
`static final int`	`HTTP_UNAUTHORIZED`	HTTP Status-Code 401: Unauthorized.
`static final int`	`HTTP_UNAVAILABLE`	HTTP Status-Code 503: Service Unavailable.
`static final int`	`HTTP_UNSUPPORTED_TYPE`	HTTP Status-Code 415: Unsupported Media Type.
`static final int`	`HTTP_USE_PROXY`	HTTP Status-Code 305: Use Proxy.
`static final int`	`HTTP_VERSION`	HTTP Status-Code 505: HTTP Version Not Supported.
`protected boolean`	`instanceFollowRedirects`	If `true`, the protocol will automatically follow redirects.
`protected String`	`method`	The HTTP method (GET,POST,PUT,etc.).
`protected int`	`responseCode`	An `int` representing the three digit HTTP Status-Code.
`protected String`	`responseMessage`	The HTTP response message.

Fields declared in class java.net.URLConnection

allowUserInteraction, connected, doInput, doOutput, ifModifiedSince, url, useCaches

Constructor Summary

HttpURLConnection(URL u)

Modifier	Constructor	Description
`protected`	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 `boolean` indicating whether or not HTTP redirects (3xx) should be automatically followed.
`String`	`getHeaderField(int n)`	Returns the value for the `n`^th header field.
`String`	`getHeaderFieldKey(int n)`	Returns the key for the `n`^th header field.
`boolean`	`getInstanceFollowRedirects()`	Returns the value of this `HttpURLConnection`'s `instanceFollowRedirects` field.
`Permission`	`getPermission()`	Returns a `SocketPermission` object representing the permission necessary to connect to the destination host and port.
`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 `Authenticator` to be used when authentication is requested through the HTTP protocol for this `HttpURLConnection`.
`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 `HttpURLConnection` instance.
`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 Details

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

Deprecated.

it is misplaced and shouldn't have existed.

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 Details

HttpURLConnection

protected HttpURLConnection(URL u)

Constructor for the HttpURLConnection.

Parameters:: u - the URL

Method Details

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 of HttpURLConnection which support supplying an Authenticator for a specific HttpURLConnection 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 for HttpURLConnection instances which share the same Authenticator instance, and authentication information, if cached, may only be reused for an HttpURLConnection sharing that same Authenticator.
Parameters:: auth - The Authenticator that should be used by this HttpURLConnection.
Throws:: UnsupportedOperationException - if setting an Authenticator is not supported by the underlying implementation.; IllegalStateException - if URLConnection is already connected.; NullPointerException - if the supplied auth is null.
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 class URLConnection
Parameters:: n - an index, where n >=0.
Returns:: the key for the n^th header field, or null 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.

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.

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 class URLConnection

Parameters:

n - an index, where n>=0.

Returns:

the value of the n^th header field, or null 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 - a boolean indicating whether or not to follow HTTP redirects.

Throws:

SecurityException - if a security manager exists and its checkSetFactory method doesn't allow the operation.

See Also:

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 - a boolean 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's instanceFollowRedirects 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

are legal, subject to protocol restrictions. The default method is GET.

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 Unauthorized

It 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 Found

Extracts 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. This method returns true if the connection is known to be going or has gone through proxies, and returns false if the connection will never go through a proxy or if the use of a proxy cannot be determined.

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 class URLConnection
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.

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https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/net/HttpURLConnection.html