Java Doc for URLConnection.java in  » 6.0-JDK-Modules » j2me » java » net » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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.

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 getContent, 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.ietf.org/rfc/rfc2068.txt
 

Ths default is "sticky", being a part of the static state of all URLConnections.

Ths default is "sticky", being a part of the static state of all URLConnections.

If called on a connection that sets the same header multiple times with possibly different values, only the last value is returned.
Parameters:
  name - the name of a header field.

This form of getHeaderField exists because some connection types (e.g., http-ng) have pre-parsed headers.

If there is a security manager, this method first calls the security manager's checkSetFactory method to ensure the operation 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:   java.net.URLConnection.getAllowUserInteraction
See Also:   java.net.URLConnection.setAllowUserInteraction(boolean)
See Also:   java.net.URLConnection.setDefaultAllowUserInteraction(boolean)

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:   java.net.URLConnection.getDoInput
See Also:   java.net.URLConnection.setDoInput(boolean)

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:   java.net.URLConnection.getDoOutput
See Also:   java.net.URLConnection.setDoOutput(boolean)

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:   java.net.URLConnection.getIfModifiedSince
See Also:   java.net.URLConnection.setIfModifiedSince(long)

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:   java.net.URLConnection.getURL
See Also:   java.net.URLConnection.url

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:   java.net.URLConnection.setUseCaches(boolean)
See Also:   java.net.URLConnection.getUseCaches
See Also:   java.net.URLConnection.setDefaultUseCaches(boolean)

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.
exception:
  IOException - if an I/O error occurs while opening theconnection.
See Also:   java.net.URLConnection.connected
See Also:   

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.

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. the default value of the allowUserInteractionfield.
See Also:   URLConnection.setDefaultAllowUserInteraction(boolean)

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. the default value of a URLConnection'suseCaches flag.
See Also:   URLConnection.setDefaultUseCaches(boolean)

If called on a connection that sets the same header multiple times with possibly different values, only the last value is returned.
Parameters:
  name - the name of a header field. the value of the named header field, or nullif there is no such field in the header.

This method can be used in conjunction with the URLConnection.getHeaderFieldKey(int) getHeaderFieldKey method to iterate through all the headers in the message.
Parameters:
  n - an index, where n>=0 the value of the n^th header fieldor null if there are fewer than n+1 fields
See Also:   java.net.URLConnection.getHeaderFieldKey(int)

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.
Parameters:
  Default - a default value. the value of the field, parsed as a date. The value of theDefault argument is returned if the field ismissing or malformed.

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.
Parameters:
  Default - the default value. the value of the named field, parsed as an integer. TheDefault value is returned if the field ismissing or malformed.

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. the permission object representing the permissionnecessary to make the connection represented by thisURLConnection.
exception:
  IOException - if the computation of the permissionrequires network or file I/O and an exception occurs whilecomputing it.

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. a guess at the content type, or null if nonecan be determined.
exception:
  IOException - if an I/O error occurs while reading theinput stream.
See Also:   java.io.InputStream.mark(int)
See Also:   java.io.InputStream.markSupported
See Also:   java.net.URLConnection.getContentType

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.
exception:
  Error - if the factory has already been defined.
exception:
  SecurityException - if a security manager exists and its checkSetFactory method doesn't allow the operation.
See Also:   java.net.ContentHandlerFactory
See Also:   java.net.URLConnection.getContent
See Also:   SecurityManager.checkSetFactory

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.
Parameters:
  doinput - the new value.
throws:
  IllegalStateException - if already connected
See Also:   java.net.URLConnection.doInput
See Also:   URLConnection.getDoInput()

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:
  dooutput - the new value.
throws:
  IllegalStateException - if already connected
See Also:   URLConnection.getDoOutput()

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
exception:
  SecurityException - if a security manager exists and its checkSetFactory method doesn't allow the operation.
See Also:   SecurityManager.checkSetFactory
See Also:   URLConnection.getFileNameMap()
since:
   1.2

NOTE: HTTP requires all request properties which can legally have multiple instances with the same key to use a comma-seperated list syntax which enables multiple properties to be appended into a single property.
Parameters:
  key - the keyword by which the request is known(e.g., "accept").
Parameters:
  value - the value associated with it.
throws:
  IllegalStateException - if already connected
throws:
  NullPointerException - if key is null
See Also:   URLConnection.getRequestProperty(java.lang.String)

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.
Parameters:
  usecaches - a boolean indicating whether or not to allow caching
throws:
  IllegalStateException - if already connected
See Also:   URLConnection.getUseCaches()