URLConnection

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## java.net
Class URLConnection

    java.lang.Object
      java.net.URLConnection

Direct Known Subclasses:
  HttpURLConnection, JarURLConnection

    public abstract class URLConnection

extends Object

The abstract class `URLConnection` is the superclass of all classes that represent a communications link between the application and a URL. Instances of this class can be used both to read from and to write to the resource referenced by the URL. In general, creating a connection to a URL is a multistep process:

`openConnection()` | `connect()`
--- | ---
Manipulate parameters that affect the connection to the remote resource. | Interact with the resource; query header fields and contents.

---------------------------->
time

1. The connection object is created by invoking the `openConnection` method on a URL.
2. The setup parameters and general request properties are manipulated.
3. The actual connection to the remote object is made, using the `connect` method.
4. The remote object becomes available. The header fields and the contents of the remote object can be accessed.

The setup parameters are modified using the following methods:

- `setAllowUserInteraction`
- `setDoInput`
- `setDoOutput`
- `setIfModifiedSince`
- `setUseCaches`

and the general request properties are modified using the method:

- `setRequestProperty`

Default values for the `AllowUserInteraction` and `UseCaches` parameters can be set using the methods `setDefaultAllowUserInteraction` and `setDefaultUseCaches`.

Each of the above `set` methods has a corresponding `get` method to retrieve the value of the parameter or general request property. The specific parameters and general request properties that are applicable are protocol specific.

The following methods are used to access the header fields and the contents after the connection is made to the remote object:

- `getContent`
- `getHeaderField`
- `getInputStream`
- `getOutputStream`

Certain header fields are accessed frequently. The methods:

- `getContentEncoding`
- `getContentLength`
- `getContentType`
- `getDate`
- `getExpiration`
- `getLastModifed`

provide convenient access to these fields. The `getContentType` method is used by the `getContent` method to determine the type of the remote object; subclasses may find it convenient to override the `getContentType` method.

In the common case, all of the pre-connection parameters and general request properties can be ignored: the pre-connection parameters and request properties default to sensible values. For most clients of this interface, there are only two interesting methods: `getInputStream` and `getContent`, which are mirrored in the `URL` class by convenience methods.

More information on the request properties and header fields of an `http` connection can be found at:

>      http://www.ietf.org/rfc/rfc2068.txt

Note about `fileNameMap`: In versions prior to JDK 1.1.6, field `fileNameMap` of `URLConnection` was public. In JDK 1.1.6 and later, `fileNameMap` is private; accessor and mutator methods `getFileNameMap` and `setFileNameMap` are added to access it. This change is also described on the Compatibility page. Calling 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:
  JDK1.0
Version:
  1.75, 05/03/00
Author:
  James Gosling
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)`

Method Summary
`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.
`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.
`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.
`boolean` | `getDefaultUseCaches()` Returns the default value of a `URLConnection`'s `useCaches` flag.
`boolean` | `getDoInput()` Returns the value of this `URLConnection`'s `doInput` flag.
`boolean` | `getDoOutput()` Returns the value of this `URLConnection`'s `doOutput` flag.
`long` | `getExpiration()` Returns the value of the `expires` header field.
`static FileNameMap` | `getFileNameMap()` 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 Default)` Returns the value of the named field parsed as date.
`int` | `getHeaderFieldInt(String name, int Default)` Returns the value of the named field parsed as a number.
`String` | `getHeaderFieldKey(int n)` Returns the key for the `n`th header field.
`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.
`Map` | `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`.
`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.
`void` | `setDefaultUseCaches(boolean defaultusecaches)` Sets the default value of the `useCaches` field to the specified value.
`void` | `setDoInput(boolean doinput)` Sets the value of the `doInput` field for this `URLConnection` to the specified value.
`void` | `setDoOutput(boolean dooutput)` Sets the value of the `doOutput` field for this `URLConnection` to the specified value.
`static void` | `setFileNameMap(FileNameMap map)` Sets the FileNameMap.
`void` | `setIfModifiedSince(long ifmodifiedsince)` Sets the value of the `ifModifiedSince` field of this `URLConnection` to the specified value.
`void` | `setRequestProperty(String key, String value)` Sets the general request property.
`void` | `setUseCaches(boolean usecaches)` Sets the value of the `useCaches` field of this `URLConnection` to the specified value.
`String` | `toString()` Returns a `String` representation of this URL connection.

Methods inherited from class java.lang.Object
`equals, getClass, hashCode, notify, notifyAll, wait, wait, wait`

Method Detail

### 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 at lib/content-types.properties under java home.

  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:
    `IOException` - if an I/O error occurs while opening the connection.
  See Also:
    `connected`

### getURL

    public URL getURL()

  Returns the value of this `URLConnection`'s `URL` field.

  Returns:
    the value of this `URLConnection`'s `URL` field.
  See Also:
    `url`

### getContentLength

    public int getContentLength()

  Returns the value of the `content-length` header field.

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

### getContentType

    public String getContentType()

  Returns the value of the `content-type` header field.

  Returns:
    the content type of the resource that the URL references, or `null` if not known.
  See Also:
    `getHeaderField(java.lang.String)`

### getContentEncoding

    public String getContentEncoding()

  Returns the value of the `content-encoding` header field.

  Returns:
    the content encoding of the resource that the URL references, or `null` if not known.
  See Also:
    `getHeaderField(java.lang.String)`

### getExpiration

    public long getExpiration()

  Returns the value of the `expires` header field.

  Returns:
    the expiration date of the resource that this URL references, or 0 if not known. The value is the number of milliseconds since January 1, 1970 GMT.
  See Also:
    `getHeaderField(java.lang.String)`

### getDate

    public long getDate()

  Returns the value of the `date` header field.

  Returns:
    the sending date of the resource that the URL references, or `0` if not known. The value returned is the number of milliseconds since January 1, 1970 GMT.
  See Also:
    `getHeaderField(java.lang.String)`

### getLastModified

    public long getLastModified()

  Returns the value of the `last-modified` header field. The result is the number of milliseconds since January 1, 1970 GMT.

  Returns:
    the date the resource referenced by this `URLConnection` was last modified, or 0 if not known.
  See Also:
    `getHeaderField(java.lang.String)`

### getHeaderField

    public String getHeaderField(String name)

  Returns the 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 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.

  Returns:
    a Map of header fields
  Since:
    1.4

### getHeaderFieldInt

    public int getHeaderFieldInt(String name,
                                 int Default)

  Returns the value of the named field parsed as a number.

  This form of `getHeaderField` exists because some connection types (e.g., `http-ng`) have pre-parsed headers. Classes for that connection type can override this method and short-circuit the parsing.

  Parameters:
    `name` - the name of the header field.
    `Default` - the default value.
  Returns:
    the value of the named field, parsed as an integer. The `Default` value is returned if the field is missing or malformed.

### getHeaderFieldDate

    public long getHeaderFieldDate(String name,
                                   long Default)

  Returns the value of the named field parsed as date. The result is the number of milliseconds since January 1, 1970 GMT represented by the named field.

  This form of `getHeaderField` exists because some connection types (e.g., `http-ng`) have pre-parsed headers. Classes for that connection type can override this method and short-circuit the parsing.

  Parameters:
    `name` - the name of the header field.
    `Default` - a default value.
  Returns:
    the value of the field, parsed as a date. The value of the `Default` argument is returned if the field is missing or malformed.

### getHeaderFieldKey

    public String getHeaderFieldKey(int n)

  Returns the key for the `n`th header field. 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.

### 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:

  1. If the application has set up a content handler factory instance using the `setContentHandlerFactory` method, the `createContentHandler` method of that instance is called with the content type as an argument; the result is a content handler for that content type.
  2. If no content handler factory has yet been set up, or if the factory's `createContentHandler` method returns `null`, then the application loads the class named:

     >              sun.net.www.content.<contentType>

     where <contentType> is formed by taking the content-type string, replacing all slash characters with a `period` ('.'), and all other non-alphanumeric characters with the underscore character '`_`'. The alphanumeric characters are specifically the 26 uppercase ASCII letters '`A`' through '`Z`', the 26 lowercase ASCII letters '`a`' through '`z`', and the 10 ASCII digits '`0`' through '`9`'. If the specified class does not exist, or is not a subclass of `ContentHandler`, then an `UnknownServiceException` is thrown.

  Returns:
    the object fetched. The `instanceof` 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.
  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 a connection to the URL. For example, a `URLConnection` representing a `file:` URL would return a `java.io.FilePermission` object.

  The permission returned may dependent upon the state of the connection. For example, the permission before connecting may be different from that after connecting. For example, an HTTP sever, say foo.com, may redirect the connection to a different host, say bar.com. Before connecting the permission returned by the connection will represent the permission needed to connect to foo.com, while the permission returned after connecting will be to bar.com.

  Permissions are generally used for two purposes: to protect caches of objects obtained through URLConnections, and to check the right of a recipient to learn about a particular URL. In the first case, the permission should be obtained after the object has been obtained. For example, in an HTTP connection, this will represent the permission to connect to the host from which the data was ultimately fetched. In the second case, the permission should be obtained and tested before connecting.

  Returns:
    the permission object representing the permission necessary to make the connection represented by this URLConnection.
  Throws:
    `IOException` - if the computation of the permission requires network or file I/O and an exception occurs while computing it.

### getInputStream

    public InputStream getInputStream()
                               throws IOException

  Returns an input stream that reads from this open connection.

  Returns:
    an input stream that reads from this open connection.
  Throws:
    `IOException` - if an I/O error occurs while creating the input stream.
    `UnknownServiceException` - if the protocol does not support input.

### getOutputStream

    public OutputStream getOutputStream()
                                 throws IOException

  Returns an output stream that writes to this connection.

  Returns:
    an output stream that writes to this connection.
  Throws:
    `IOException` - if an I/O error occurs while creating the output stream.
    `UnknownServiceException` - if the protocol does not support output.

### toString

    public String toString()

  Returns a `String` representation of this URL connection.

  Overrides:
    `toString` in class `Object`
  Returns:
    a string representation of this `URLConnection`.

### setDoInput

    public void setDoInput(boolean doinput)

  Sets the value of the `doInput` field for this `URLConnection` to the specified value.

  A URL connection can be used for input and/or output. Set the DoInput flag to true if you intend to use the URL connection for input, false if not. The default is true.

  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.

  Ths default is "sticky", being a part of the static state of all URLConnections. This flag applies to the next, and all following URLConnections that are created.

  Returns:
    the default value of the `allowUserInteraction` field.
  See Also:
    `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.

  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.

  Ths default is "sticky", being a part of the static state of all URLConnections. This flag applies to the next, and all following URLConnections that are created.

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

### setDefaultUseCaches

    public void setDefaultUseCaches(boolean defaultusecaches)

  Sets the default value of the `useCaches` field to the specified value.

  Parameters:
    `defaultusecaches` - the new value.
  See Also:
    `getDefaultUseCaches()`

### 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-seperated list syntax which enables multiple properties to be appended into a single property.

  Parameters:
    `key` - the keyword by which the request is known (e.g., "`accept`").
    `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.

  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(java.lang.String)`

### 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 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.

  Returns:
    a Map of the general request properties for this connection.
  Throws:
    `IllegalStateException` - if already connected
  Since:
    1.4

### 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()`

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