HttpURLConnection, JarURLConnection
public abstract class URLConnection extends Object
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 method on a URL. connect method. 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 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 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:
Invoking thehttp://www.ietf.org/rfc/rfc2616.txt
close() methods on the InputStream or OutputStream of an URLConnection after a request may free network resources associated with this instance, unless particular protocol specifications specify different behaviours for it.| Modifier and Type | Field | Description |
|---|---|---|
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. |
| Modifier | Constructor | Description |
|---|---|---|
protected |
Constructs a URL connection to the specified URL. |
| Modifier and Type | Method | Description |
|---|---|---|
void |
addRequestProperty |
Adds a general request property specified by a key-value pair. |
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. |
int |
getConnectTimeout() |
Returns setting for connect timeout. |
Object |
getContent() |
Retrieves the contents of this URL connection. |
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. |
long |
getContentLengthLong() |
Returns the value of the content-length header field as a long. |
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 |
Deprecated. The instance specific getRequestProperty method should be used after an appropriate instance of URLConnection is obtained. |
boolean |
getDefaultUseCaches() |
Returns the default value of a URLConnection's useCaches flag. |
static boolean |
getDefaultUseCaches |
Returns the default value of the useCaches flag for the given protocol. |
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() |
Loads filename map (a mimetable) from a data file. |
String |
getHeaderField |
Returns the value for the nth header field. |
String |
getHeaderField |
Returns the value of the named header field. |
long |
getHeaderFieldDate |
Returns the value of the named field parsed as date. |
int |
getHeaderFieldInt |
Returns the value of the named field parsed as a number. |
String |
getHeaderFieldKey |
Returns the key for the nth header field. |
long |
getHeaderFieldLong |
Returns the value of the named field parsed as a number. |
Map |
getHeaderFields() |
Returns an unmodifiable Map of the header fields. |
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. |
int |
getReadTimeout() |
Returns setting for read timeout. 0 return implies that the option is disabled (i.e., timeout of infinity). |
Map |
getRequestProperties() |
Returns an unmodifiable Map of general request properties for this connection. |
String |
getRequestProperty |
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. |
static String |
guessContentTypeFromName |
Tries to determine the content type of an object, based on the specified "file" component of a URL. |
static String |
guessContentTypeFromStream |
Tries to determine the type of an input stream based on the characters at the beginning of the input stream. |
void |
setAllowUserInteraction |
Set the value of the allowUserInteraction field of this URLConnection. |
void |
setConnectTimeout |
Sets a specified timeout value, in milliseconds, to be used when opening a communications link to the resource referenced by this URLConnection. |
static void |
setContentHandlerFactory |
Sets the ContentHandlerFactory of an application. |
static void |
setDefaultAllowUserInteraction |
Sets the default value of the allowUserInteraction field for all future URLConnection objects to the specified value. |
static void |
setDefaultRequestProperty |
Deprecated. The instance specific setRequestProperty method should be used after an appropriate instance of URLConnection is obtained. |
void |
setDefaultUseCaches |
Sets the default value of the useCaches field to the specified value. |
static void |
setDefaultUseCaches |
Sets the default value of the useCaches field for the named protocol to the given value. |
void |
setDoInput |
Sets the value of the doInput field for this URLConnection to the specified value. |
void |
setDoOutput |
Sets the value of the doOutput field for this URLConnection to the specified value. |
static void |
setFileNameMap |
Sets the FileNameMap. |
void |
setIfModifiedSince |
Sets the value of the ifModifiedSince field of this URLConnection to the specified value. |
void |
setReadTimeout |
Sets the read timeout to a specified timeout, in milliseconds. |
void |
setRequestProperty |
Sets the general request property. |
void |
setUseCaches |
Sets the value of the useCaches field of this URLConnection to the specified value. |
String |
toString() |
Returns a String representation of this URL connection. |
protected URL url
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.
protected boolean doInput
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.
protected boolean doOutput
setDoOutput method. Its value is returned by the getDoOutput 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.
protected boolean allowUserInteraction
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.
protected boolean useCaches
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.
The default setting may be overridden per protocol with setDefaultUseCaches(String,boolean).
protected long ifModifiedSince
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.
protected boolean connected
false, this connection object has not created a communications link to the specified URL. If true, the communications link has been established.protected URLConnection(URL url)
url - the specified URL.public static FileNameMap getFileNameMap()
public static void setFileNameMap(FileNameMap map)
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.
map - the FileNameMap to be setSecurityException - if a security manager exists and its checkSetFactory method doesn't allow the operation.public abstract void connect() throws IOException
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.
SocketTimeoutException - if the timeout expires before the connection can be establishedIOException - if an I/O error occurs while opening the connection.public void setConnectTimeout(int timeout)
Some non-standard implementation of this method may ignore the specified timeout. To see the connect timeout set, please call getConnectTimeout().
timeout - an int that specifies the connect timeout value in millisecondsIllegalArgumentException - if the timeout parameter is negativepublic int getConnectTimeout()
0 return implies that the option is disabled (i.e., timeout of infinity).
int that indicates the connect timeout value in millisecondspublic void setReadTimeout(int timeout)
Some non-standard implementation of this method ignores the specified timeout. To see the read timeout set, please call getReadTimeout().
timeout - an int that specifies the timeout value to be used in millisecondsIllegalArgumentException - if the timeout parameter is negativepublic int getReadTimeout()
int that indicates the read timeout value in millisecondspublic URL getURL()
URLConnection's URL field.URLConnection's URL field.public int getContentLength()
content-length header field. Note: getContentLengthLong() should be preferred over this method, since it returns a long instead and is therefore more portable.
-1 if the content length is not known, or if the content length is greater than Integer.MAX_VALUE.public long getContentLengthLong()
content-length header field as a long.-1 if the content length is not known.public String getContentType()
content-type header field.null if not known.public String getContentEncoding()
content-encoding header field.null if not known.public long getExpiration()
expires header field.public long getDate()
date header field.0 if not known. The value returned is the number of milliseconds since January 1, 1970 GMT.public long getLastModified()
last-modified header field. The result is the number of milliseconds since January 1, 1970 GMT.URLConnection was last modified, or 0 if not known.public String getHeaderField(String name)
If called on a connection that sets the same header multiple times with possibly different values, only the last value is returned.
name - the name of a header field.null if there is no such field in the header.public Map<String,List<String>> getHeaderFields()
URLConnection. In the implementation of these methods, if a given key has multiple corresponding values, they must be returned in the order they were added, preserving the insertion-order.public int getHeaderFieldInt(String name, int defaultValue)
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.
name - the name of the header field.defaultValue - the default value.defaultValue value is returned if the field is missing or malformed.public long getHeaderFieldLong(String name, long defaultValue)
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.
name - the name of the header field.defaultValue - the default value.defaultValue value is returned if the field is missing or malformed.public long getHeaderFieldDate(String name, long defaultValue)
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.
name - the name of the header field.defaultValue - a default value.defaultValue argument is returned if the field is missing or malformed.public String getHeaderFieldKey(int n)
nth header field. Some implementations may treat the 0th header field as special, in which case, getHeaderField(0) may return some value, but getHeaderFieldKey(0) returns null. For n > 0 it returns null if there are fewer than n+1 fields.n - an index, where n>=0
nth header field, or null if there are fewer than n+1 fields when n > 0.public String getHeaderField(int n)
nth header field. It returns null if there are fewer than n+1 fields. This method can be used in conjunction with the getHeaderFieldKey method to iterate through all the headers in the message.
n - an index, where n>=0
nth header field or null if there are fewer than n+1 fieldspublic Object getContent() throws IOException
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.
This is done as follows:
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. ContentHandlerFactory has yet been set up, or if the factory's createContentHandler method returns null, then the ServiceLoader mechanism is used to locate ContentHandlerFactory implementations using the system class loader. The order that factories are located is implementation specific, and an implementation is free to cache the located factories. A ServiceConfigurationError, Error or RuntimeException thrown from the createContentHandler, if encountered, will be propagated to the calling thread. The
createContentHandler method of each factory, if instantiated, is invoked, with the content type, until a factory returns non-null, or all factories have been exhausted. ContentHandler. If the class does not exist, or is not a subclass of
ContentHandler, then an UnknownServiceException is thrown. instanceof operator should be used to determine the specific kind of object returned.IOException - if an I/O error occurs while getting the content.UnknownServiceException - if the protocol does not support the content type.public Object getContent(Class<?>[] classes) throws IOException
classes - the Class array indicating the requested typesinstanceof operator should be used to determine the specific kind of object returned.IOException - if an I/O error occurs while getting the content.UnknownServiceException - if the protocol does not support the content type.public Permission getPermission() throws IOException
java.security.AllPermission. Subclasses should override this method and return the permission that best represents the permission required to make 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.
IOException - if the computation of the permission requires network or file I/O and an exception occurs while computing it.public InputStream getInputStream() throws IOException
InputStream returned by this method can wrap an InflaterInputStream, whose read(byte[], int, int) method can modify any element of the output buffer.IOException - if an I/O error occurs while creating the input stream.UnknownServiceException - if the protocol does not support input.public OutputStream getOutputStream() throws IOException
IOException - if an I/O error occurs while creating the output stream.UnknownServiceException - if the protocol does not support output.public String toString()
String representation of this URL connection.public void setDoInput(boolean doinput)
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.
doinput - the new value.IllegalStateException - if already connectedpublic boolean getDoInput()
URLConnection's doInput flag.URLConnection's doInput flag.public void setDoOutput(boolean dooutput)
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.
dooutput - the new value.IllegalStateException - if already connectedpublic boolean getDoOutput()
URLConnection's doOutput flag.URLConnection's doOutput flag.public void setAllowUserInteraction(boolean allowuserinteraction)
allowUserInteraction field of this URLConnection.allowuserinteraction - the new value.IllegalStateException - if already connectedpublic boolean getAllowUserInteraction()
allowUserInteraction field for this object.allowUserInteraction field for this object.public static void setDefaultAllowUserInteraction(boolean defaultallowuserinteraction)
allowUserInteraction field for all future URLConnection objects to the specified value.defaultallowuserinteraction - the new value.public static boolean getDefaultAllowUserInteraction()
allowUserInteraction field. This 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.
allowUserInteraction field.public void setUseCaches(boolean usecaches)
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. A default value can also be set per-protocol using setDefaultUseCaches(String,boolean).
usecaches - a boolean indicating whether or not to allow cachingIllegalStateException - if already connectedpublic boolean getUseCaches()
URLConnection's useCaches field.URLConnection's useCaches field.public void setIfModifiedSince(long ifmodifiedsince)
ifModifiedSince field of this URLConnection to the specified value.ifmodifiedsince - the new value.IllegalStateException - if already connectedpublic long getIfModifiedSince()
ifModifiedSince field.ifModifiedSince field.public boolean getDefaultUseCaches()
URLConnection's useCaches flag. This 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. This default value can be over-ridden per protocol using setDefaultUseCaches(String,boolean)
URLConnection's useCaches flag.public void setDefaultUseCaches(boolean defaultusecaches)
useCaches field to the specified value. This default value can be over-ridden per protocol using setDefaultUseCaches(String,boolean)
defaultusecaches - the new value.public static void setDefaultUseCaches(String protocol, boolean defaultVal)
useCaches field for the named protocol to the given value. This value overrides any default setting set by setDefaultUseCaches(boolean) for the given protocol. Successive calls to this method change the setting and affect the default value for all future connections of that protocol. The protocol name is case insensitive.protocol - the protocol to set the default fordefaultVal - whether caching is enabled by default for the given protocolpublic static boolean getDefaultUseCaches(String protocol)
useCaches flag for the given protocol. If setDefaultUseCaches(String,boolean) was called for the given protocol, then that value is returned. Otherwise, if setDefaultUseCaches(boolean) was called, then that value is returned. If neither method was called, the return value is true. The protocol name is case insensitive.protocol - the protocol whose defaultUseCaches setting is requireduseCaches flag for the given protocol.public void setRequestProperty(String key, String value)
NOTE: HTTP requires all request properties which can legally have multiple instances with the same key to use a comma-separated list syntax which enables multiple properties to be appended into a single property.
key - the keyword by which the request is known (e.g., "Accept").value - the value associated with it.IllegalStateException - if already connectedNullPointerException - if key is null
public void addRequestProperty(String key, String value)
key - the keyword by which the request is known (e.g., "Accept").value - the value associated with it.IllegalStateException - if already connectedNullPointerException - if key is nullpublic String getRequestProperty(String key)
key - the keyword by which the request is known (e.g., "Accept").IllegalStateException - if already connectedpublic Map<String,List<String>> getRequestProperties()
addRequestProperty(String, String) method, these values will be returned in the order they were added. This method must preserve the insertion order of such values. The default implementation of this method preserves the insertion order when multiple values are added for a given key. The values are returned in the order they were added.IllegalStateException - if already connected@Deprecated public static void setDefaultRequestProperty(String key, String value)
URLConnection is created, it is initialized with these properties.key - the keyword by which the request is known (e.g., "Accept").value - the value associated with the key.@Deprecated public static String getDefaultRequestProperty(String key)
key - the keyword by which the request is known (e.g., "Accept").public static void setContentHandlerFactory(ContentHandlerFactory fac)
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.
fac - the desired factory.Error - if the factory has already been defined.SecurityException - if a security manager exists and its checkSetFactory method doesn't allow the operation.public static String guessContentTypeFromName(String fname)
getContentType method.fname - a filename.public static String guessContentTypeFromStream(InputStream is) throws IOException
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.
is - an input stream that supports marks.null if none can be determined.IOException - if an I/O error occurs while reading the input stream.
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/net/URLConnection.html