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java.net
Class URLConnection

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

Direct Known Subclasses:

HttpURLConnection, JarURLConnection

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

1. The connection object is created by invoking the openConnection method on a URL.
2. The setup parameters and general request properties are manipulated.
3. The actual connection to the remote object is made, using the connect method.
4. 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
• getLastModifed

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 and mutator methods getFileNameMap and setFileNameMap are added to access it. This change is also described on the JDK Compatibility page.

Since:

JDK1.0

See Also:

URL.openConnection(), connect(), getContent(), getContentEncoding(), getContentLength(), getContentType(), getDate(), getExpiration(), getHeaderField(int), getHeaderField(java.lang.String), getInputStream(), getLastModified(), getOutputStream(), setAllowUserInteraction(boolean), setDefaultRequestProperty(java.lang.String, java.lang.String), setDefaultUseCaches(boolean), setDoInput(boolean), setDoOutput(boolean), setIfModifiedSince(long), setRequestProperty(java.lang.String, java.lang.String), setUseCaches(boolean)

Field Summary

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.
protected boolean	connected If false, this connection object has not created a communications link to the specified URL.
protected boolean	doInput This variable is set by the setDoInput method.
protected boolean	doOutput This variable is set by the setDoOutput method.
protected long	ifModifiedSince Some protocols support skipping the fetching of the object unless the object has been modified more recently than a certain time.
protected URL	url The URL represents the remote object on the World Wide Web to which this connection is opened.
protected boolean	useCaches If true, the protocol is allowed to use caching whenever it can.

Constructor Summary

protected

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

Method Summary

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

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

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(boolean)

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(boolean)

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(boolean), setDefaultAllowUserInteraction(boolean)

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(boolean), getUseCaches(), setDefaultUseCaches(boolean)

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 milliseconds 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(long)

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.

Constructor Detail

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.

Method Detail

getFileNameMap

public static FileNameMap getFileNameMap()

Returns the FileNameMap.

Since:

JDK1.2

setFileNameMap

public static void setFileNameMap(FileNameMap map)

Sets the FileNameMap.

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:

map - the FileNameMap to be set

Throws:

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

Since:

JDK1.2

See Also:

SecurityManager.checkSetFactory()

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(java.lang.String)

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(java.lang.String)

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 milliseconds since January 1, 1970 GMT.

See Also:

getHeaderField(java.lang.String)

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 milliseconds since January 1, 1970 GMT.

See Also:

getHeaderField(java.lang.String)

getLastModified

public long getLastModified()

Returns the value of the last-modified header field. The result is the number of milliseconds 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(java.lang.String)

getHeaderField

public String getHeaderField(String name)

Returns the name of the specified header field.

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.

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 milliseconds 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(int)

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:

1. 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.
2. 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:

            sun.net.www.content.<contentType>
        

   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.

UnknownServiceException - if the protocol does not support the content type.

See Also:

ContentHandlerFactory.createContentHandler(java.lang.String), getContentType(), setContentHandlerFactory(java.net.ContentHandlerFactory)

getPermission

public Permission getPermission()
                         throws IOException

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 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 URLConnection representing a file: URL would return a java.io.FilePermission object.

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.

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 after 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 before connecting.

Returns:

the permission object representing the permission necessary to make the connection represented by this URLConnection.

Throws:

IOException - if the computation of the permission requires network or file I/O and an exception occurs while computing it.

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.

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.

UnknownServiceException - if the protocol does not support output.

toString

public String toString()

Returns a String representation of this URL connection.

Overrides:

toString in class Object

Returns:

a string representation of this URLConnection.

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(java.lang.String, java.lang.String)

setContentHandlerFactory

public static 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

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:

fac - the desired factory.

Throws:

Error - if the factory has already been defined.

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

See Also:

ContentHandlerFactory, getContent(), SecurityManager.checkSetFactory()

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:

InputStream.mark(int), InputStream.markSupported(), getContentType()