Module java.base
Package 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:

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`
- `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:

>      http://www.ietf.org/rfc/rfc2616.txt

Invoking the `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.

Since:
  1.0
External Specifications
  - RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1
See Also:
  - `URL.openConnection()`
  - `connect()`
  - `getContent()`
  - `getContentEncoding()`
  - `getContentLength()`
  - `getContentType()`
  - `getDate()`
  - `getExpiration()`
  - `getHeaderField(int)`
  - `getHeaderField(java.lang.String)`
  - `getInputStream()`
  - `getLastModified()`
  - `getOutputStream()`
  - `setAllowUserInteraction(boolean)`
  - `setDefaultUseCaches(boolean)`
  - `setDoInput(boolean)`
  - `setDoOutput(boolean)`
  - `setIfModifiedSince(long)`
  - `setRequestProperty(java.lang.String, java.lang.String)`
  - `setUseCaches(boolean)`

- ## Field Summary

  Fields
  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.

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `URLConnection(URL url)`
  Constructs a URL connection to the specified URL.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `addRequestProperty(String key, String value)`
  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(Class<?>[] classes)`
  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(String key)`
  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(String protocol)`
  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(int n)`
  Returns the value for the `n`th header field.
  `String`
  `getHeaderField(String name)`
  Returns the value of the named header field.
  `long`
  `getHeaderFieldDate(String name, long defaultValue)`
  Returns the value of the named field parsed as date.
  `int`
  `getHeaderFieldInt(String name, int defaultValue)`
  Returns the value of the named field parsed as a number.
  `String`
  `getHeaderFieldKey(int n)`
  Returns the key for the `n`th header field.
  `long`
  `getHeaderFieldLong(String name, long defaultValue)`
  Returns the value of the named field parsed as a number.
  `Map<String,List<String>>`
  `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<String,List<String>>`
  `getRequestProperties()`
  Returns an unmodifiable Map of general request properties for this connection.
  `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.
  `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`.
  `void`
  `setConnectTimeout(int timeout)`
  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(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)`
  Deprecated.
  The instance specific setRequestProperty method should be used after an appropriate instance of URLConnection is obtained.
  `void`
  `setDefaultUseCaches(boolean defaultusecaches)`
  Sets the default value of the `useCaches` field to the specified value.
  `static void`
  `setDefaultUseCaches(String protocol, boolean defaultVal)`
  Sets the default value of the `useCaches` field for the named protocol to the given 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`
  `setReadTimeout(int timeout)`
  Sets the read timeout to a specified timeout, in milliseconds.
  `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 declared in class java.lang.Object

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

- ## Field Details

  - ### 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 `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`.

    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.

    The default setting may be overridden per protocol with `setDefaultUseCaches(String,boolean)`.

    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 Details

  - ### 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 Details

  - ### getFileNameMap

    public static FileNameMap getFileNameMap()
    Loads filename map (a mimetable) from a data file. It will first try to load the user-specific table, defined by "content.types.user.table" property. If that fails, it tries to load the default built-in table.
    Returns:
      the FileNameMap
    Since:
      1.2
    See Also:
      - `setFileNameMap(java.net.FileNameMap)`

  - ### 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:
      1.2
    See Also:
      - `SecurityManager.checkSetFactory()`
      - `getFileNameMap()`

  - ### 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:
      `SocketTimeoutException` - if the timeout expires before the connection can be established
      `IOException` - if an I/O error occurs while opening the connection.
    See Also:
      - `connected`
      - `getConnectTimeout()`
      - `setConnectTimeout(int)`

  - ### setConnectTimeout

    public void setConnectTimeout(int timeout)
    Sets a specified timeout value, in milliseconds, to be used when opening a communications link to the resource referenced by this URLConnection. If the timeout expires before the connection can be established, a java.net.SocketTimeoutException is raised. A timeout of zero is interpreted as an infinite timeout.

    Some non-standard implementation of this method may ignore the specified timeout. To see the connect timeout set, please call getConnectTimeout().

    Parameters:
      `timeout` - an `int` that specifies the connect timeout value in milliseconds
    Throws:
      `IllegalArgumentException` - if the timeout parameter is negative
    Since:
      1.5
    See Also:
      - `getConnectTimeout()`
      - `connect()`

  - ### getConnectTimeout

    public int getConnectTimeout()
    Returns setting for connect timeout.

    0 return implies that the option is disabled (i.e., timeout of infinity).

    Returns:
      an `int` that indicates the connect timeout value in milliseconds
    Since:
      1.5
    See Also:
      - `setConnectTimeout(int)`
      - `connect()`

  - ### setReadTimeout

    public void setReadTimeout(int timeout)
    Sets the read timeout to a specified timeout, in milliseconds. A non-zero value specifies the timeout when reading from Input stream when a connection is established to a resource. If the timeout expires before there is data available for read, a java.net.SocketTimeoutException is raised. A timeout of zero is interpreted as an infinite timeout.

    Some non-standard implementation of this method ignores the specified timeout. To see the read timeout set, please call getReadTimeout().

    Parameters:
      `timeout` - an `int` that specifies the timeout value to be used in milliseconds
    Throws:
      `IllegalArgumentException` - if the timeout parameter is negative
    Since:
      1.5
    See Also:
      - `getReadTimeout()`
      - `InputStream.read()`

  - ### getReadTimeout

    public int getReadTimeout()
    Returns setting for read timeout. 0 return implies that the option is disabled (i.e., timeout of infinity).
    Returns:
      an `int` that indicates the read timeout value in milliseconds
    Since:
      1.5
    See Also:
      - `setReadTimeout(int)`
      - `InputStream.read()`

  - ### 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.

    Note: `getContentLengthLong()` should be preferred over this method, since it returns a `long` instead and is therefore more portable.

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

  - ### getContentLengthLong

    public long getContentLengthLong()
    Returns the value of the `content-length` header field as a long.
    Returns:
      the content length of the resource that this connection's URL references, or `-1` if the content length is not known.
    Since:
      1.7

  - ### 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 value of the named header field.

    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.
    Returns:
      the value of the named header field, or `null` if there is no such field in the header.

  - ### getHeaderFields

    public Map<String,List<String>> getHeaderFields()
    Returns an unmodifiable Map of the header fields. The Map keys are Strings that represent the response-header field names. Each Map value is an unmodifiable List of Strings that represents the corresponding field values. This method is overridden by the subclasses of `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.
    Implementation Requirements:
      The default implementation of this method returns an empty map always.
    Returns:
      a Map of header fields
    Since:
      1.4

  - ### getHeaderFieldInt

    public int getHeaderFieldInt(String name, int defaultValue)
    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.
      `defaultValue` - the default value.
    Returns:
      the value of the named field, parsed as an integer. The `defaultValue` value is returned if the field is missing or malformed.

  - ### getHeaderFieldLong

    public long getHeaderFieldLong(String name, long defaultValue)
    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.
      `defaultValue` - the default value.
    Returns:
      the value of the named field, parsed as a long. The `defaultValue` value is returned if the field is missing or malformed.
    Since:
      1.7

  - ### getHeaderFieldDate

    public long getHeaderFieldDate(String name, long defaultValue)
    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.
      `defaultValue` - a default value.
    Returns:
      the value of the field, parsed as a date. The value of the `defaultValue` 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. Some implementations may treat the `0`th 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.
    Parameters:
      `n` - an index, where `n>=0`
    Returns:
      the key for the `n`th header field, or `null` if there are fewer than `n+1` fields when `n > 0`.

  - ### getHeaderField

    public String getHeaderField(int n)
    Returns the value for the `n`th 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.

    Parameters:
      `n` - an index, where `n>=0`
    Returns:
      the value of the `n`th header field or `null` if there are fewer than `n+1` fields
    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.

    This is done as follows:

    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 `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.
    3. Failing that, this method tries to load a content handler class as defined by `ContentHandler`. If the class does not exist, or is not a subclass of `ContentHandler`, then an `UnknownServiceException` is thrown.
    Returns:
      the object fetched. The `instanceof` operator 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)`

  - ### getContent

    public Object getContent(Class<?>[] classes) throws IOException
    Retrieves the contents of this URL connection.
    Parameters:
      `classes` - the `Class` array indicating the requested types
    Returns:
      the object fetched that is the first match of the type specified in the classes array. null if none of the requested types are supported. The `instanceof` operator 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.
    Since:
      1.3
    See Also:
      - `getContent()`
      - `ContentHandlerFactory.createContentHandler(java.lang.String)`
      - `getContent(java.lang.Class[])`
      - `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 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. A SocketTimeoutException can be thrown when reading from the returned input stream if the read timeout expires before data is available for read.
    API Note:
      The `InputStream` returned by this method can wrap an `InflaterInputStream`, whose `read(byte[], int, int)` method can modify any element of the output buffer.
    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.
    See Also:
      - `setReadTimeout(int)`
      - `getReadTimeout()`

  - ### 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.

    Parameters:
      `doinput` - the new value.
    Throws:
      `IllegalStateException` - if already connected
    See Also:
      - `doInput`
      - `getDoInput()`

  - ### getDoInput

    public boolean getDoInput()
    Returns the value of this `URLConnection`'s `doInput` flag.
    Returns:
      the value of this `URLConnection`'s `doInput` flag.
    See Also:
      - `setDoInput(boolean)`

  - ### 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:
      `dooutput` - the new value.
    Throws:
      `IllegalStateException` - if already connected
    See Also:
      - `getDoOutput()`

  - ### getDoOutput

    public boolean getDoOutput()
    Returns the value of this `URLConnection`'s `doOutput` flag.
    Returns:
      the value of this `URLConnection`'s `doOutput` flag.
    See Also:
      - `setDoOutput(boolean)`

  - ### setAllowUserInteraction

    public void setAllowUserInteraction(boolean allowuserinteraction)
    Set the value of the `allowUserInteraction` field of this `URLConnection`.
    Parameters:
      `allowuserinteraction` - the new value.
    Throws:
      `IllegalStateException` - if already connected
    See Also:
      - `getAllowUserInteraction()`

  - ### 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:
      - `setAllowUserInteraction(boolean)`

  - ### 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:
      - `getDefaultAllowUserInteraction()`

  - ### getDefaultAllowUserInteraction

    public static boolean getDefaultAllowUserInteraction()
    Returns the default value of the `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.

    Returns:
      the default value of the `allowUserInteraction` field.
    See Also:
      - `setDefaultAllowUserInteraction(boolean)`

  - ### 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. A default value can also be set per-protocol using `setDefaultUseCaches(String,boolean)`.

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

  - ### getUseCaches

    public boolean getUseCaches()
    Returns the value of this `URLConnection`'s `useCaches` field.
    Returns:
      the value of this `URLConnection`'s `useCaches` field.
    See Also:
      - `setUseCaches(boolean)`

  - ### setIfModifiedSince

    public void setIfModifiedSince(long ifmodifiedsince)
    Sets the value of the `ifModifiedSince` field of this `URLConnection` to the specified value.
    Parameters:
      `ifmodifiedsince` - the new value.
    Throws:
      `IllegalStateException` - if already connected
    See Also:
      - `getIfModifiedSince()`

  - ### getIfModifiedSince

    public long getIfModifiedSince()
    Returns the value of this object's `ifModifiedSince` field.
    Returns:
      the value of this object's `ifModifiedSince` field.
    See Also:
      - `setIfModifiedSince(long)`

  - ### getDefaultUseCaches

    public boolean getDefaultUseCaches()
    Returns the default value of a `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)`

    Returns:
      the default value of a `URLConnection`'s `useCaches` flag.
    See Also:
      - `setDefaultUseCaches(boolean)`

  - ### setDefaultUseCaches

    public void setDefaultUseCaches(boolean defaultusecaches)
    Sets the default value of the `useCaches` field to the specified value. This default value can be over-ridden per protocol using `setDefaultUseCaches(String,boolean)`
    Parameters:
      `defaultusecaches` - the new value.
    See Also:
      - `getDefaultUseCaches()`

  - ### setDefaultUseCaches

    public static void setDefaultUseCaches(String protocol, boolean defaultVal)
    Sets the default value of the `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.
    Parameters:
      `protocol` - the protocol to set the default for
      `defaultVal` - whether caching is enabled by default for the given protocol
    Since:
      9

  - ### getDefaultUseCaches

    public static boolean getDefaultUseCaches(String protocol)
    Returns the default value of the `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.
    Parameters:
      `protocol` - the protocol whose defaultUseCaches setting is required
    Returns:
      the default value of the `useCaches` flag for the given protocol.
    Since:
      9

  - ### setRequestProperty

    public void setRequestProperty(String key, String value)
    Sets the general request property. If a property with the key already exists, overwrite its value with the new 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.

    Parameters:
      `key` - the keyword by which the request is known (e.g., "`Accept`").
      `value` - the value associated with it.
    Throws:
      `IllegalStateException` - if already connected
      `NullPointerException` - if key is `null`
    See Also:
      - `getRequestProperty(java.lang.String)`

  - ### addRequestProperty

    public void addRequestProperty(String key, String value)
    Adds a general request property specified by a key-value pair. This method will not overwrite existing values associated with the same key. This method could be a no-op if appending a value to the map is not supported by the protocol being used in a given subclass.
    Parameters:
      `key` - the keyword by which the request is known (e.g., "`Accept`").
      `value` - the value associated with it.
    Throws:
      `IllegalStateException` - if already connected
      `NullPointerException` - if key is null
    Since:
      1.4
    See Also:
      - `getRequestProperties()`

  - ### getRequestProperty

    public String getRequestProperty(String key)
    Returns the value of the named general request property for this connection.
    Parameters:
      `key` - the keyword by which the request is known (e.g., "Accept").
    Returns:
      the value of the named general request property for this connection. If key is null, then null is returned.
    Throws:
      `IllegalStateException` - if already connected
    See Also:
      - `setRequestProperty(java.lang.String, java.lang.String)`

  - ### getRequestProperties

    public Map<String,List<String>> getRequestProperties()
    Returns an unmodifiable Map of general request properties for this connection. The Map keys are Strings that represent the request-header field names. Each Map value is a unmodifiable List of Strings that represents the corresponding field values. If multiple values for a given key are added via the `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.
    Returns:
      a Map of the general request properties for this connection.
    Throws:
      `IllegalStateException` - if already connected
    Since:
      1.4

  - ### setDefaultRequestProperty

    @Deprecated public static void setDefaultRequestProperty(String key, String value)
    Deprecated.
    The instance specific setRequestProperty method should be used after an appropriate instance of URLConnection is obtained. Invoking this method will have no effect.
    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.
    See Also:
      - `setRequestProperty(java.lang.String,java.lang.String)`
      - `getDefaultRequestProperty(java.lang.String)`

  - ### getDefaultRequestProperty

    @Deprecated public static String getDefaultRequestProperty(String key)
    Deprecated.
    The instance specific getRequestProperty method should be used after an appropriate instance of URLConnection is obtained.
    Returns the value of the default request property. Default request properties are set for every connection.
    Parameters:
      `key` - the keyword by which the request is known (e.g., "Accept").
    Returns:
      the value of the default request property for the specified key.
    See Also:
      - `getRequestProperty(java.lang.String)`
      - `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

    public 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()`