Class java.net.URLConnection - JDK 5 Documentation v1.1.8, Java 2 SDK 英文文档

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Class java.net.URLConnection

java.lang.Object
   |
   +----java.net.URLConnection

public abstract class URLConnection
extends Object

The abstract class 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:

`openConnection()`	`connect()`
Manipulate parameters that affect the connection to the remote resource.	Interact with the resource; query header fields and contents.

---------------------------->
time

The connection object is created by invoking the openConnection method on a URL.
The setup parameters and general request properties are manipulated.
The actual connection to the remote object is made, using the connect method.
The remote object becomes available. The header fields and the contents of the remote object can be accessed.

The setup parameters are modified using the following methods:


   setAllowUserInteraction
   
setDoInput
   
setDoOutput
   
setIfModifiedSince

setUseCaches

and the general request properties are modified using the method:

setRequestProperty

Default values for the AllowUserInteraction and UseCaches parameters can be set using the methods setDefaultAllowUserInteraction and setDefaultUseCaches. Default values for general request properties can be set using the setDefaultRequestProperty method.

Each of the above set methods has a corresponding 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.

The following methods are used to access the header fields and the contents after the connection is made to the remote object:


   getContent
   
getHeaderField
   
getInputStream

getOutputStream

Certain header fields are accessed frequently. The methods:


   getContentEncoding
   
getContentLength
   
getContentType
   
getDate
   
getExpiration

getLastModified

provide convenient access to these fields. The getContentType method is used by the getContent method to determine the type of the remote object; subclasses may find it convenient to override the getContentType method.

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: getInputStream and getObject, which are mirrored in the URL class by convenience methods.

More information on the request properties and header fields of an http connection can be found at:


   http://www.w3.org/hypertext/WWW/Protocols/HTTP1.0/draft-ietf-http-spec.html

Note about fileNameMap: In versions prior to JDK 1.1.6, field fileNameMap of URLConnection was public. In JDK 1.1.6 and later, fileNameMap is private; accessor method getFileNameMap() and mutator method setFileNameMap(FileNameMap) are added to access it. This change is also described on the JDK 1.1.x Compatibility page.

See Also:: openConnection, connect, getContent, getContentEncoding, getContentLength, getContentType, getDate, getExpiration, getHeaderField, getHeaderField, getInputStream, getLastModified, getOutputStream, setAllowUserInteraction, setDefaultRequestProperty, setDefaultUseCaches, setDoInput, setDoOutput, setIfModifiedSince, setRequestProperty, setUseCaches

allowUserInteraction: If true, this URL is being examined in a context in which it makes sense to allow user interactions such as popping up an authentication dialog.
connected: If false, this connection object has not created a communications link to the specified URL.
doInput: This variable is set by the setDoInput method.
doOutput: This variable is set by the setDoOutput method.
ifModifiedSince: Some protocols support skipping the fetching of the object unless the object has been modified more recently than a certain time.
url: The URL represents the remote object on the World Wide Web to which this connection is opened.
useCaches: If true, the protocol is allowed to use caching whenever it can.

URLConnection(URL): Constructs a URL connection to the specified URL.

connect(): Opens a communications link to the resource referenced by this URL, if such a connection has not already been established.
getAllowUserInteraction(): Returns the value of the allowUserInteraction field for this object.
getContent(): Retrieves the contents of this URL connection.
getContentEncoding(): Returns the value of the content-encoding header field.
getContentLength(): Returns the value of the content-length header field.
getContentType(): Returns the value of the content-type header field.
getDate(): Returns the value of the date header field.
getDefaultAllowUserInteraction(): Returns the default value of the allowUserInteraction field.
getDefaultRequestProperty(String): Returns the value of the default request property.
getDefaultUseCaches(): Returns the default value of a URLConnection's useCaches flag.
getDoInput(): Returns the value of this URLConnection's doInput flag.
getDoOutput(): Returns the value of this URLConnection's doOutput flag.
getExpiration(): Returns the value of the expires header field.
getFileNameMap(): Returns the FileNameMap.
getHeaderField(int): Returns the value for the n^th header field.
getHeaderField(String): Returns the value of the specified header field.
getHeaderFieldDate(String, long): Returns the value of the named field parsed as date.
getHeaderFieldInt(String, int): Returns the value of the named field parsed as a number.
getHeaderFieldKey(int): Returns the key for the n^th header field.
getIfModifiedSince(): Returns the value of this object's ifModifiedSince field.
getInputStream(): Returns an input stream that reads from this open connection.
getLastModified(): Returns the value of the last-modified header field.
getOutputStream(): Returns an output stream that writes to this connection.
getRequestProperty(String): Returns the value of the named general request property for this connection.
getURL(): Returns the value of this URLConnection's URL field.
getUseCaches(): Returns the value of this URLConnection's useCaches field.
guessContentTypeFromName(String): Tries to determine the content type of an object, based on the specified "file" component of a URL.
guessContentTypeFromStream(InputStream): Tries to determine the type of an input stream based on the characters at the beginning of the input stream.
setAllowUserInteraction(boolean): Set the value of the allowUserInteraction field of this URLConnection.
setContentHandlerFactory(ContentHandlerFactory): Sets the ContentHandlerFactory of an application.
setDefaultAllowUserInteraction(boolean): Sets the default value of the allowUserInteraction field for all future URLConnection objects to the specified value.
setDefaultRequestProperty(String, String): Sets the default value of a general request property.
setDefaultUseCaches(boolean): Sets the default value of the useCaches field to the specified value.
setDoInput(boolean): Sets the value of the doInput field for this URLConnection to the specified value.
setDoOutput(boolean): Sets the value of the doOutput field for this URLConnection to the specified value.
setFileNameMap(FileNameMap): Sets the FileNameMap.
setIfModifiedSince(long): Sets the value of the ifModifiedSince field of this URLConnection to the specified value.
setRequestProperty(String, String): Sets the general request property.
setUseCaches(boolean): Sets the value of the useCaches field of this URLConnection to the specified value.
toString(): Returns a String representation of this URL connection.

url

 protected URL url

The URL represents the remote object on the World Wide Web to which this connection is opened.

The value of this field can be accessed by the getURL method.

The default value of this variable is the value of the URL argument in the URLConnection constructor.

See Also:: getURL, url

doInput

 protected boolean doInput

This variable is set by the setDoInput method. Its value is returned by the getDoInput method.

A URL connection can be used for input and/or output. Setting the doInput flag to true indicates that the application intends to read data from the URL connection.

The default value of this field is true.

See Also:: getDoInput, setDoInput

doOutput

 protected boolean doOutput

This variable is set by the setDoOutput method. Its value is returned by the getDoInput method.

A URL connection can be used for input and/or output. Setting the doOutput flag to true indicates that the application intends to write data to the URL connection.

The default value of this field is false.

See Also:: getDoOutput, setDoOutput

allowUserInteraction

 protected boolean allowUserInteraction

If true, this URL is being examined in a context in which it makes sense to allow user interactions such as popping up an authentication dialog. If false, then no user interaction is allowed.

The value of this field can be set by the setAllowUserInteraction method. Its value is returned by the getAllowUserInteraction method. Its default value is the value of the argument in the last invocation of the setDefaultAllowUserInteraction method.

See Also:: getAllowUserInteraction, setAllowUserInteraction, setDefaultAllowUserInteraction

useCaches

 protected boolean useCaches

If true, the protocol is allowed to use caching whenever it can. If false, the protocol must always try to get a fresh copy of the object.

This field is set by the setUseCaches method. Its value is returned by the getUseCaches method.

Its default value is the value given in the last invocation of the setDefaultUseCaches method.

See Also:: setUseCaches, getUseCaches, setDefaultUseCaches

ifModifiedSince

 protected long ifModifiedSince

Some protocols support skipping the fetching of the object unless the object has been modified more recently than a certain time.

A nonzero value gives a time as the number of seconds since January 1, 1970, GMT. The object is fetched only if it has been modified more recently than that time.

This variable is set by the setIfModifiedSince method. Its value is returned by the getIfModifiedSince method.

The default value of this field is 0, indicating that the fetching must always occur.

See Also:: getIfModifiedSince, setIfModifiedSince

connected

 protected boolean connected

If false, this connection object has not created a communications link to the specified URL. If true, the communications link has been established.

URLConnection

 protected URLConnection(URL url)

Constructs a URL connection to the specified URL. A connection to the object referenced by the URL is not created.

Parameters:: url - the specified URL.

getFileNameMap

 public static FileNameMap getFileNameMap()

Returns the FileNameMap.

Returns:: s the FileNameMap

setFileNameMap

 public static void setFileNameMap(FileNameMap map)

Sets the FileNameMap.

Parameters:: map - the FileNameMap to be set

connect

 public abstract void connect() throws IOException

Opens a communications link to the resource referenced by this URL, if such a connection has not already been established.

If the connect method is called when the connection has already been opened (indicated by the connected field having the value true), the call is ignored.

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: IOException: if an I/O error occurs while opening the connection.
See Also:: connected

getURL

 public URL getURL()

Returns the value of this URLConnection's URL field.

Returns:: the value of this URLConnection's URL field.
See Also:: url

getContentLength

 public int getContentLength()

Returns the value of the content-length header field.

Returns:: the content length of the resource that this connection's URL references, or -1 if the content length is not known.

getContentType

 public String getContentType()

Returns the value of the content-type header field.

Returns:: the content type of the resource that the URL references, or null if not known.
See Also:: getHeaderField

getContentEncoding

 public String getContentEncoding()

Returns the value of the content-encoding header field.

Returns:: the content encoding of the resource that the URL references, or null if not known.
See Also:: getHeaderField

getExpiration

 public long getExpiration()

Returns the value of the expires header field.

Returns:: the expiration date of the resource that this URL references, or 0 if not known. The value is the number of seconds since January 1, 1970 GMT.
See Also:: getHeaderField

getDate

 public long getDate()

Returns the value of the date header field.

Returns:: the sending date of the resource that the URL references, or 0 if not known. The value returned is the number of seconds since January 1, 1970 GMT.
See Also:: getHeaderField

getLastModified

 public long getLastModified()

Returns the value of the last-modified header field. The result is the number of seconds since January 1, 1970 GMT.

Returns:: the date the resource referenced by this URLConnection was last modified, or 0 if not known.
See Also:: getHeaderField

getHeaderField

 public String getHeaderField(String name)

Returns the value of the specified header field. Names of header fields to pass to this method can be obtained from getHeaderFieldKey.

Parameters:: name - the name of a header field.
Returns:: the value of the named header field, or null if there is no such field in the header.
See Also:: getHeaderFieldKey

getHeaderFieldInt

 public int getHeaderFieldInt(String name,
                              int Default)

Returns the value of the named field parsed as a number.

This form of getHeaderField exists because some connection types (e.g., http-ng) have pre-parsed headers. Classes for that connection type can override this method and short-circuit the parsing.

Parameters:: name - the name of the header field.; Default - the default value.
Returns:: the value of the named field, parsed as an integer. The Default value is returned if the field is missing or malformed.

getHeaderFieldDate

 public long getHeaderFieldDate(String name,
                                long Default)

Returns the value of the named field parsed as date. The result is the number of seconds since January 1, 1970 GMT represented by the named field.

This form of getHeaderField exists because some connection types (e.g., http-ng) have pre-parsed headers. Classes for that connection type can override this method and short-circuit the parsing.

Parameters:: name - the name of the header field.; Default - a default value.
Returns:: the value of the field, parsed as a date. The value of the Default argument is returned if the field is missing or malformed.

getHeaderFieldKey

 public String getHeaderFieldKey(int n)

Returns the key for the n^th header field.

Parameters:: n - an index.
Returns:: the key for the n^th header field, or null if there are fewer than n fields.

getHeaderField

 public String getHeaderField(int n)

Returns the value for the n^th header field. It returns null if there are fewer than n fields.

This method can be used in conjunction with the getHeaderFieldKey method to iterate through all the headers in the message.

Parameters:: n - an index.
Returns:: the value of the n^th header field.
See Also:: getHeaderFieldKey

getContent

 public Object getContent() throws IOException

Retrieves the contents of this URL connection.

This method first determines the content type of the object by calling the 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:

If the application has set up a content handler factory instance using the setContentHandlerFactory method, the createContentHandler method of that instance is called with the content type as an argument; the result is a content handler for that content type.
If no content handler factory has yet been set up, or if the factory's createContentHandler method returns null, then the application loads the class named:
where <contentType> is formed by taking the content-type string, replacing all slash characters with a period ('.'), and all other non-alphanumeric characters with the underscore character '_'. The alphanumeric characters are specifically the 26 uppercase ASCII letters 'A' through 'Z', the 26 lowercase ASCII letters 'a' through 'z', and the 10 ASCII digits '0' through '9'. If the specified class does not exist, or is not a subclass of ContentHandler, then an UnknownServiceException is thrown.

Returns:: the object fetched. The instanceOf operation should be used to determine the specific kind of object returned.
Throws: IOException: if an I/O error occurs while getting the content.
Throws: UnknownServiceException: if the protocol does not support the content type.
See Also:: createContentHandler, getContentType, setContentHandlerFactory

getInputStream

 public InputStream getInputStream() throws IOException

Returns an input stream that reads from this open connection.

Returns:: an input stream that reads from this open connection.
Throws: IOException: if an I/O error occurs while creating the input stream.
Throws: UnknownServiceException: if the protocol does not support input.

getOutputStream

 public OutputStream getOutputStream() throws IOException

Returns an output stream that writes to this connection.

Returns:: an output stream that writes to this connection.
Throws: IOException: if an I/O error occurs while creating the output stream.
Throws: UnknownServiceException: if the protocol does not support output.

toString

 public String toString()

Returns a String representation of this URL connection.

Returns:: a string representation of this URLConnection.
Overrides:: toString in class Object

setDoInput

 public void setDoInput(boolean doinput)

Sets the value of the doInput field for this URLConnection to the specified value.

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 unless DoOutput is explicitly set to true, in which case DoInput defaults to false.

Parameters:: value - the new value.
See Also:: doInput

getDoInput

 public boolean getDoInput()

Returns the value of this URLConnection's doInput flag.

Returns:: the value of this URLConnection's doInput flag.
See Also:: doInput

setDoOutput

 public void setDoOutput(boolean dooutput)

Sets the value of the doOutput field for this URLConnection to the specified value.

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.

Parameters:: value - the new value.
See Also:: doOutput

getDoOutput

 public boolean getDoOutput()

Returns the value of this URLConnection's doOutput flag.

Returns:: the value of this URLConnection's doOutput flag.
See Also:: doOutput

setAllowUserInteraction

 public void setAllowUserInteraction(boolean allowuserinteraction)

Set the value of the allowUserInteraction field of this URLConnection.

Parameters:: allowuserinteraction - the new value.
See Also:: allowUserInteraction

getAllowUserInteraction

 public boolean getAllowUserInteraction()

Returns the value of the allowUserInteraction field for this object.

Returns:: the value of the allowUserInteraction field for this object.
See Also:: allowUserInteraction

setDefaultAllowUserInteraction

 public static void setDefaultAllowUserInteraction(boolean defaultallowuserinteraction)

Sets the default value of the allowUserInteraction field for all future URLConnection objects to the specified value.

Parameters:: defaultallowuserinteraction - the new value.
See Also:: allowUserInteraction

getDefaultAllowUserInteraction

 public static boolean getDefaultAllowUserInteraction()

Returns the default value of the allowUserInteraction field.

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.

Returns:: the default value of the allowUserInteraction field.
See Also:: allowUserInteraction

setUseCaches

 public void setUseCaches(boolean usecaches)

Sets the value of the useCaches field of this URLConnection to the specified value.

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.

See Also:: useCaches

getUseCaches

 public boolean getUseCaches()

Returns the value of this URLConnection's useCaches field.

Returns:: the value of this URLConnection's useCaches field.
See Also:: useCaches

setIfModifiedSince

 public void setIfModifiedSince(long ifmodifiedsince)

Sets the value of the ifModifiedSince field of this URLConnection to the specified value.

Parameters:: value - the new value.
See Also:: ifModifiedSince

getIfModifiedSince

 public long getIfModifiedSince()

Returns the value of this object's ifModifiedSince field.

Returns:: the value of this object's ifModifiedSince field.
See Also:: ifModifiedSince

getDefaultUseCaches

 public boolean getDefaultUseCaches()

Returns the default value of a URLConnection's useCaches flag.

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.

Returns:: the default value of a URLConnection's useCaches flag.
See Also:: useCaches

setDefaultUseCaches

 public void setDefaultUseCaches(boolean defaultusecaches)

Sets the default value of the useCaches field to the specified value.

Parameters:: defaultusecaches - the new value.
See Also:: useCaches

setRequestProperty

 public void setRequestProperty(String key,
                                String value)

Sets the general request property.

Parameters:: key - the keyword by which the request is known (e.g., "accept").; value - the value associated with it.

getRequestProperty

 public String getRequestProperty(String key)

Returns the value of the named general request property for this connection.

Returns:: the value of the named general request property for this connection.

setDefaultRequestProperty

 public static void setDefaultRequestProperty(String key,
                                              String value)

Sets the default value of a general request property. When a URLConnection is created, it is initialized with these properties.

Parameters:: key - the keyword by which the request is known (e.g., "accept").; value - the value associated with the key.

getDefaultRequestProperty

 public static String getDefaultRequestProperty(String key)

Returns the value of the default request property. Default request properties are set for every connection.

Returns:: the value of the default request property for the specified key.
See Also:: setDefaultRequestProperty

setContentHandlerFactory

 public static synchronized void setContentHandlerFactory(ContentHandlerFactory fac)

Sets the ContentHandlerFactory of an application. It can be called at most once by an application.

The ContentHandlerFactory instance is used to construct a content handler from a content type

Parameters:: fac - the desired factory.
Throws: Error: if the factory has already been defined.
See Also:: ContentHandlerFactory, getContent

guessContentTypeFromName

 protected static String guessContentTypeFromName(String fname)

Tries to determine the content type of an object, based on the specified "file" component of a URL. This is a convenience method that can be used by subclasses that override the getContentType method.

Parameters:: fname - a filename.
Returns:: a guess as to what the content type of the object is, based upon its file name.
See Also:: getContentType

guessContentTypeFromStream

 public static String guessContentTypeFromStream(InputStream is) throws IOException

Tries to determine the type of an input stream based on the characters at the beginning of the input stream. This method can be used by subclasses that override the getContentType method.

Ideally, this routine would not be needed. But many http servers return the incorrect content type; in addition, there are many nonstandard extensions. Direct inspection of the bytes to determine the content type is often more accurate than believing the content type claimed by the http server.

Parameters:: is - an input stream that supports marks.
Returns:: a guess at the content type, or null if none can be determined.
Throws: IOException: if an I/O error occurs while reading the input stream.
See Also:: mark, markSupported, getContentType

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