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Java Fundamental Classes Reference

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The java.net Package
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URLConnection

Name

URLConnection

Synopsis

Class Name:

java.net.URLConnection

Superclass:

java.lang.Object

Immediate Subclasses:

java.net.HttpURLConnection

Interfaces Implemented:

None

Availability:

JDK 1.0 or later

Description

The URLConnection class is an abstract class that represents a connection to a URL. A subclass of URLConnection supports a protocol-specific connection. A URLConnection can both read from and write to a URL.

A URLConnection object is created when the openConnection() method is called for a URL object. At this point, the actual connection has not yet been made, so setup parameters and general request properties can be modified for the specific connection. The various set methods control the setup parameters and request properties. Then the actual connection is made by calling the connect() method. Finally, the remote object becomes available, and the header fields and the content are accessed using various get methods.

The URLConnection class defines quite a few methods for setting parameters and retrieving information. Fortunately, for simple connections, all of the setup parameters and request properties can be left alone, as they all have reasonable default values. In most cases, you'll only be interested in the getInputStream() and getContent() methods. getInputStream() provides an InputStream that reads from the URL, while getContent() uses a ContentHandler to return an Object that represents the content of the resource. These methods are mirrored by the openStream() and getContent() convenience methods in the URL class.

Class Summary

public abstract class java.net.URLConnection extends java.lang.Object {
  // Variables
  protected boolean allowUserInteraction;
  protected boolean connected;
  protected boolean doInput;
  protected boolean doOutput;
  public static FileNameMap fileNameMap;            // New in 1.1
  protected long ifModifiedSince;
  protected URL url;
  protected boolean useCaches;
  // Constructors
  protected URLConnection(URL url);
  // Class Methods
  public static boolean getDefaultAllowUserInteraction();
  public static String getDefaultRequestProperty(String key);
  protected static String guessContentTypeFromName(String fname);
  public static String guessContentTypeFromStream(InputStream is);
  public static synchronized void setContentHandlerFactory(
                                  ContentHandlerFactory fac);
  public static void setDefaultAllowUserInteraction(
                     boolean defaultallowuserinteraction);
  public static void setDefaultRequestProperty(String key, 
                     String value);
  // Instance Methods
  public abstract void connect();
  public boolean getAllowUserInteraction();
  public Object getContent()
  public String getContentEncoding();
  public int getContentLength();
  public String getContentType();
  public long getDate();
  public boolean getDefaultUseCaches();
  public boolean getDoInput();
  public boolean getDoOutput();
  public long getExpiration();
  public String getHeaderField(int n);
  public String getHeaderField(String name);
  public long getHeaderFieldDate(String name, long default);
  public int getHeaderFieldInt(String name, int default);
  public String getHeaderFieldKey(int n);
  public long getIfModifiedSince();
  public InputStream getInputStream();
  public long getLastModified();
  public OutputStream getOutputStream();
  public String getRequestProperty(String key);
  public URL getURL();
  public boolean getUseCaches();
  public void setAllowUserInteraction(boolean allowuserinteraction);
  public void setDefaultUseCaches(boolean defaultusecaches);
  public void setDoInput(boolean doinput);
  public void setDoOutput(boolean dooutput);
  public void setIfModifiedSince(long ifmodifiedsince);
  public void setRequestProperty(String key, String value);
  public void setUseCaches(boolean usecaches);
  public String toString();
}

Variables

allowUserInteraction

protected boolean allowUserInteraction

Description

A flag that indicates whether or not user interaction is enabled for this connection. If this variable is true, it is possible to allow user interactions such as popping up dialog boxes. If it is false, no user interaction is allowed. The variable can be set by setAllowUserInteraction() and retrieved by getAllowUserInteraction(). The default value is false, unless the setDefaultAllowUserInteraction() method has been called, in which case that method call controls the default value.

connected

protected boolean connected

Description

A flag that indicates whether or not this object has established a connection to a remote host.

doInput

protected boolean doInput

Description

A flag that indicates whether or not this connection is enabled for input. Setting this variable to true indicates that the connection is going to read data. The variable can be set by setDoInput() and retrieved by getDoInput(). The default value is true.

doOutput

protected boolean doOutput

Description

A flag that indicates whether or not this connection is enabled for output. Setting this variable to true indicates that the connection is going to write data. The variable can be set by setDoOutput() and retrieved by getDoOutput(). The default value is false.

fileNameMap

public static FileNameMap fileNameMap

Availability

New as of JDK 1.1

Description

A reference to the object that maps filename extensions to MIME type strings. This variable is used in guessContentTypeFromName().

ifModifiedSince

protected long ifModifiedSince

Description

A time value, specified as the number of seconds since January 1, 1970, that controls whether or not a resource is fetched based on its last modification time. Some protocols do not bother to retrieve a resource if there is a current local cached copy. However, if the resource has been modified more recently than ifModifiedSince, it is retrieved. If ifModifiedSince is 0, the resource is always retrieved. The variable can be set by setIfModifiedSince() and retrieved by getIfModifiedSince(). The default value is 0, which means that the resource must always be retrieved.

url

protected URL url

Description

The resource to which this URLConnection connects. This variable is set to the value of the URL argument in the URLConnection constructor. It can be retrieved by getURL().

useCaches

protected boolean useCaches

Description

A flag that indicates whether or not locally cached resources are used. Some protocols cache documents. If this variable is true, the protocol is allowed to use caching whenever possible. If it is false, the protocol must always try to retrieve the resource. The variable can be set by setUseCaches() and retrieved by getUseCaches(). The default value is true, unless the setDefaultUseCaches() method has been called, in which case that method call controls the default value.

Constructors

URLConnection

protected URLConnection(URL url)

Parameters

url

The URL to access.

Description

This constructor creates a URLConnection object to manage a connection to the destination specified by the given URL. The actual connection is not created, however.

Class Methods

getDefaultAllowUserInteraction

public static boolean getDefaultAllowUserInteraction()

Returns

true if user interaction is allowed by default; false otherwise.

Description

This method returns the default value of the allowUserInteraction variable for all instances of URLConnection. The default value is false, unless the setDefaultAllowUserInteraction() method has been called, in which case that method call controls the default value.

getDefaultRequestProperty

public static String getDefaultRequestProperty(String key)

Parameters

key

The name of a request property.

Returns

The default value of the named request property.

Description

This method returns the default value for the request property named by the key parameter.

guessContentTypeFromName

protected static String guessContentTypeFromName(String fname)

Parameters

fname

A String that contains the name of a file.

Returns

A guess at the MIME type of the given file, or null if a guess cannot be made.

Description

This method uses the FileNameMap specified by the variable fileNameMap to return a MIME content type for the given filename.

guessContentTypeFromStream

 protected static String guessContentTypeFromStream(InputStream is) throws IOException 

Parameters

is

The input stream to inspect

Returns

A guess at the MIME type of the given input stream, or null if a guess cannot be made.

Throws

IOException

If any kind of I/O error occurs.

Description

This method looks at the first few bytes of an InputStream and returns a guess of the MIME content type. Note that the InputStream must support marks, which usually means that there is a BufferedInputStream at some level. Here are some strings that are recognized and the inferred content type:

String MIME Type Guess

#def

image/x-bitmap

<!

text/html

<body

text/html

<head>

text/html

<html>

text/html

! XPM2

image/x-pixmap

GIF8

image/gif

setContentHandlerFactory

 public static synchronized void setContentHandlerFactory(ContentHandlerFactory fac) 

Parameters

fac

An object that implements ContentHandlerFactory.

Throws

Error

If the factory has already been defined.

SecurityException

If the application does not have permission to set the factory.

Description

This method tells the URLConnection class to use the given ContentHandlerFactory object for all URLConnection objects. The purpose of this mechanism is to allow a program that hosts applets, such as a web browser, control over the creation of ContentHandler objects.

setDefaultAllowUserInteraction

 public static void setDefaultAllowUserInteraction( boolean defaultallowuserinteraction) 

Parameters

defaultallowuserinteraction

The new default value.

Description

This method sets the default value of the allowUserInteraction variable for all new instances of URLConnection.

setDefaultRequestProperty

 public static void setDefaultRequestProperty(String key, String value) 

Parameters

key

The name of a request property.

value

The new default value.

Description

This method sets the default value of the request property named by the key parameter.

Instance Methods

connect

public abstract void connect() throws IOException

Throws

IOException

If any kind of I/O error occurs.

Description

When a URLConnection object is created, it is not immediately connected to the resource specified by its associated URL object. This method actually establishes the connection. If this method is called after the connection has been established, it does nothing.

Before the connection is established, various parameters can be set with the set methods. After the connection has been established, it is an error to try to set these parameters.

As they retrieve information about the resource specified by the URL object, many of the get methods require that the connection be established. If the connection has not been established when one of these methods is called, the connection is established implicitly.

getAllowUserInteraction

public boolean getAllowUserInteraction()

Returns

true if user interaction is allowed for this connection; false otherwise.

Description

This method returns the value of the allowUserInteraction variable for this URLConnection.

getContent

 public Object getContent() throws IOException, UnknownServiceException 

Returns

An Object created from this URLConnection.

Throws

IOException

If any kind of I/O error occurs.

UnknownServiceException

If the protocol cannot support the content type.

Description

This method retrieves the content of the resource specified by the URL object associated with this URLConnection. If the connection for this object has not been established, the method implicitly establishes it.

The method returns an object that encapsulates the content of the connection. For example, for a connection that has content type image/jpeg, the method might return a object that belongs to subclass of Image, or for content type text/plain, it might return a String. The instanceof operator should determine the specific type of object that is returned.

The method first determines the content type of the connection by calling getContentType(). If this is the first time the content type has been seen, a content handler of type ContentHandler is created for the content type. Here are the steps that are taken to create a content handler:

  1. If an application has set up a ContentHandlerFactory by calling setContentHandlerFactory(), the method calls the createContentHandler() method of that object to create the content handler. The content type is passed as a String argument to that method.

  2. If no ContentHandlerFactory has been established, or the createContentHandler() method returns null, the method retrieves the value of the system property java.content.handler.pkgs. If this value is not null, it is interpreted as a list of packages separated by vertical bar (|) characters. The method then tries to load the class named package.contentType, where package is the name of the first package in the list and contentType is formed by taking the content-type string and replacing every slash (/) character with a period (.) and every other nonalphanumeric character with an underscore ( _ ). If the class exists and is a subclass of ContentHandler, it is used as the ContentHandler for the content type. If the class does not exist, or if it exists but is not a subclass of ContentHandler, the method tries the next package in the list.

  3. If the previous step fails to find an appropriate content handler, the method tries to load the class named sun.net.www.content.contentType, where contentType is formed by taking the content-type string and replacing every slash (/) character with a period (.) and every other nonalphanumeric character with an underscore ( _ ). If the class exists and is a subclass of ContentHandler, it is used as the ContentHandler for the content type. If the class does not exist, or if it exists but is not a subclass of ContentHandler, a UnknownServiceException is thrown.

getContentEncoding

public String getContentEncoding()

Returns

The content encoding, or null if it is not known.

Description

This method retrieves the content encoding of the resource specified by the URL object associated with this URLConnection. In other words, the method returns the value of the content-encoding header field. If the connection for this object has not been established, the method implicitly establishes it.

getContentLength

public int getContentLength()

Returns

The content length or -1 if it is not known.

Description

This method retrieves the content length of the resource specified by the URL object associated with this URLConnection. In other words, the method returns the value of the content-length header field. If the connection for this object has not been established, the method implicitly establishes it.

getContentType

public String getContentType()

Returns

The content type, or null if it is not known.

Description

This method retrieves the content type of the resource specified by the URL object associated with this URLConnection. In other words, the method returns the value of the content-type header field. This string is generally be a MIME type, such as image/jpeg or text/html. If the connection for this object has not been established, the method implicitly establishes it.

getDate

public long getDate()

Returns

The content date, or 0 if it is not known.

Description

This method retrieves the date of the resource specified by the URL object associated with this URLConnection. In other words, the method returns the value of the date header field. The date is returned as the number of seconds since January 1, 1970. If the connection for this object has not been established, the method implicitly establishes it.

getDefaultUseCaches

public boolean getDefaultUseCaches()

Returns

true if the use of caches is allowed by default; false otherwise.

Description

This method returns the default value of the useCaches variable for all instances of URLConnection. The default value is true, unless the setDefaultUseCaches() method has been called, in which case that method call controls the default value.

getDoInput

public boolean getDoInput()

Returns

true if this URLConnection is to be used for input; false otherwise.

Description

This method returns the value of the doInput variable for this URLConnection.

getDoOutput

public boolean getDoOutput()

Returns

true if this URLConnection is to be used for output; false otherwise.

Description

This method returns the value of the doOutput variable for this URLConnection.

getExpiration

public long getExpiration()

Returns

The content expiration date, or if it is not known.

Description

This method retrieves the expiration date of the resource specified by the URL object associated with this URLConnection. In other words, the method returns the value of the expires header field. The date is returned as the number of seconds since January 1, 1970. If the connection for this object has not been established, the method implicitly establishes it.

getHeaderField

public String getHeaderField(int n)

Parameters

n

A header field index.

Returns

The value of the header field with the given index, or null if n is greater than the number of fields in the header.

Description

This method retrieves the value of the header field at index n of the resource specified by the URL object associated with this URLConnection. If the connection for this object has not been established, the method implicitly establishes it.

public String getHeaderField(String name)

Parameters

name

A header field name.

Returns

The value of the named header field, or null if the header field is not known or its value cannot be determined.

Description

This method retrieves the value of the named header field of the resource specified by the URL object associated with this URLConnection. This method is a helper for methods like getContentEncoding() and getContentType(). If the connection for this object has not been established, the method implicitly establishes it.

getHeaderFieldDate

public long getHeaderFieldDate(String name, long default)

Parameters

name

A header field name.

default

A default date value.

Returns

The value of the named header field parsed as a date value, or default if the field is missing or cannot be parsed.

Description

This method retrieves the value of the named header field of the resource specified by the URL object associated with this URLConnection and parses it as a date value. The date is returned as the number of seconds since January 1, 1970. If the value of the header field cannot be determined or it is not a properly formed date, the given default value is returned. If the connection for this object has not been established, the method implicitly establishes it.

getHeaderFieldInt

public int getHeaderFieldInt(String name, int default)

Parameters

name

A header field name.

default

A default value.

Returns

The value of the named header field parsed as a number, or default if the field is missing or cannot be parsed.

Description

This method retrieves the value of the named header field of the resource specified by the URL object associated with this URLConnection and parses it as a number. If the value of the header field cannot be determined or it is not a properly formed integer, the given default value is returned. If the connection for this object has not been established, the method implicitly establishes it.

getHeaderFieldKey

public String getHeaderFieldKey(int n)

Parameters

n

A header field index.

Returns

The name of the header field at the given index, or null if n is greater than the number of fields in the header.

Description

This method retrieves the name of the header field at index n of the resource specified by the URL object associated with this URLConnection. If the connection for this object has not been established, the method implicitly establishes it.

getIfModifiedSince

public long getIfModifiedSince()

Returns

The value of the ifModifiedSince variable.

Description

This method returns the value of the ifModifiedSince variable for this URLConnection.

getInputStream

 public InputStream getInputStream() throws IOException, UnknownServiceException 

Returns

An InputStream that reads data from this connection.

Throws

IOException

If any kind of I/O error occurs.

UnknownServiceException

If this protocol does not support input.

Description

This method returns an InputStream that reads the contents of the resource specified by the URL object associated with this URLConnection.

getLastModified

public long getLastModified()

Returns

The content last-modified date, or if it is not known.

Description

This method retrieves the last-modified date of the resource specified by the URL object associated with this URLConnection. In other words, the method returns the value of the last-modified header field. The date is returned as the number of seconds since January 1, 1970. If the connection for this object has not been established, the method implicitly establishes it.

getOutputStream

 public OutputStream getOutputStream() throws IOException, UnknownServiceException 

Returns

An OutputStream that writes data to this connection.

Throws

IOException

If any kind of I/O error occurs.

UnknownServiceException

If this protocol does not support output.

Description

This method returns an OutputStream that writes to the resource specified by the URL object associated with this URLConnection.

getRequestProperty

public String getRequestProperty(String key)

Parameters

key

The name of a request property.

Returns

The value of the named request property.

Description

This method returns the value of the request property named by the key parameter.

getURL

public URL getURL()

Returns

The URL that this connection accesses.

Description

This method returns a reference to the URL associated with this object. This is the value of the url variable for this URLConnection.

getUseCaches

public boolean getUseCaches()

Returns

true if this URLConnection uses caches; false otherwise.

Description

This method returns the value of the useCaches variable for this URLConnection.

setAllowUserInteraction

public void setAllowUserInteraction(boolean allowuserinteraction)

Parameters

allowuserinteraction

A boolean value that indicates whether user interaction is allowed or not.

Throws

IllegalAccessError

If this method is called after the connection has been established.

Description

This method sets the value of the allowUserInteraction variable for this URLConnection. This method must be called before this object establishes a connection.

setDefaultUseCaches

public void setDefaultUseCaches(boolean defaultusecaches)

Parameters

defaultusecaches

The new default value.

Description

This method sets the default value of the useCaches variable for all new instances of URLConnection.

setDoInput

public void setDoInput(boolean doinput)

Parameters

doinput

A boolean value that indicates if this connection is to be used for input.

Throws

IllegalAccessError

If this method is called after the connection has been established.

Description

This method sets the value of the doInput variable for this URLConnection. This method must be called before this object establishes a connection.

setDoOutput

public void setDoOutput(boolean dooutput)

Parameters

dooutput

A boolean value that indicates if this connection is to be used for output.

Throws

IllegalAccessError

If this method is called after the connection has been established.

Description

This method sets the value of the doOutput variable for this URLConnection. This method must be called before this object establishes a connection.

setIfModifiedSince

public void setIfModifiedSince(long ifmodifiedsince)

Parameters

ifmodifiedsince

A time value, specified as the number of seconds since January 1, 1970.

Throws

IllegalAccessError

If this method is called after the connection has been established.

Description

This method sets the value of the ifModifiedSince variable for this URLConnection. This method must be called before this object establishes a connection.

setRequestProperty

public void setRequestProperty(String key, String value)

Parameters

key

The name of a request property.

value

The new value.

Throws

IllegalAccessError

If this method is called after the connection has been established.

Description

This method sets the value of the request property named by the key parameter.

setUseCaches

public void setUseCaches(boolean defaultusecaches)

Parameters

defaultusecaches

A boolean value that indicates if this connection uses caches.

Throws

IllegalAccessError

If this method is called after the connection has been established.

Description

This method sets the value of the useCaches variable for this URLConnection. This method must be called before this object establishes a connection.

toString

public String toString()

Returns

A string representation of the URLConnection.

Overrides

Object.toString()

Description

This method returns a string representation of this URLConnection.

Inherited Methods

Method Inherited From Method Inherited From

clone()

Object

equals(Object)

Object

finalize()

Object

getClass()

Object

hashCode()

Object

notify()

Object

notifyAll()

Object

wait()

Object

wait(long)

Object

wait(long, int)

Object

See Also

ContentHandler, ContentHandlerFactory, Error, FileNameMap, HttpURLConnection, IllegalAccessError, InputStream, IOException, OutputStream, SecurityException, UnknownServiceException, URL, URLStreamHandler, URLStreamHandlerFactory


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