java-networking

URL Connections

The URLConnection class represents a connection to a URL and provides methods for interacting with the resource.

It is a four step process :

Construct a URL object.

 URL url = new URL("http://www.example.com");

Open a connection to the URL.

  URLConnection connection = url.openConnection();

Get an input stream for reading from the URL.

 InputStream in = connection.getInputStream();

Read from the input stream and close the connection.

 BufferedReader reader = new BufferedReader(new InputStreamReader(in));
 String line;
 while ((line = reader.readLine()) != null) {
     System.out.println(line);
 }
 reader.close();

Methods of URLConnection class

This class represents a connection to a URL and provides methods for interacting with the resource.

public void connect(): Opens a connection to the URL.
public InputStream getInputStream(): Returns an input stream for reading from the URL.
public OutputStream getOutputStream(): Returns an output stream for writing to the URL.
public void setDoInput(boolean doinput): Sets whether input is allowed on the connection.
public void setDoOutput(boolean dooutput): Sets whether output is allowed on the connection.
public void setRequestProperty(String key, String value): Sets a request property on the connection.
public void addRequestProperty(String key, String value): Adds a request property to the connection.
public void setConnectTimeout(int timeout): Sets the connection timeout.
public void setReadTimeout(int timeout): Sets the read timeout.
public void setIfModifiedSince(long ifmodifiedsince): Sets the if-modified-since header field.

Reading the Header

i. Retrieving specific header fields

public String getContentType(): Returns the content type of the resource.
public int getContentLength(): Returns the content length of the resource.
public int getContentEncoding(): Returns the content encoding of the resource.
public long getDate(): Returns the date of the resource.
public long getExpiration(): Returns the expiration date of the resource.
public long getLastModified(): Returns the last modified date of the resource.

String contentType = connection.getContentType();
int contentLength = connection.getContentLength();
long date = connection.getDate();
long expiration = connection.getExpiration();
long lastModified = connection.getLastModified();

ii. Retrieving arbitrary header fields

public String getHeaderField(String name): Returns the value of the specified header field.
public Map<String, List<String>> getHeaderFields(): Returns a map of the header fields.
public String getHeaderFieldKey(int n): Returns the key of the header field at the specified index.

// Print all the header fields

try{
    URL url = new URL("http://www.example.com");
    URLConnection connection = url.openConnection();
    for(int i=1;  ;i++){
        String headerFieldKey = connection.getHeaderFieldKey(i);

        if(headerFieldKey == null){
            break;
        }

            System.out.println(headerField + ": " + connection.getHeaderField(headerFieldKey));
 

    }
} catch(Exception e){
    e.printStackTrace();
}

Caches

The URLConnection class provides methods for controlling the cache behavior of the connection.

Pages accessed with GET over HTTP can and will be cached by default. The setUseCaches() method can be used to disable caching.

A page accessed with POST will not be cached by default.

public void setUseCaches(boolean usecaches): Sets whether caching is allowed on the connection.
public boolean getUseCaches(): Returns whether caching is allowed on the connection.
public void setDefaultUseCaches(boolean defaultusecaches): Sets the default value for whether caching is allowed on the connection.
public static void setDefaultUseCaches(boolean defaultusecaches): Sets the default value for whether caching is allowed on all connections.
public static boolean getDefaultUseCaches(): Returns the default value for whether caching is allowed on all connections.

try{
    URL url = new URL("http://www.example.com");
    URLConnection connection = url.openConnection();
    connection.setUseCaches(false);
    InputStream in = connection.getInputStream();
    // Read from the input stream
    in.close();
} catch(Exception e){
    e.printStackTrace();
}

HttpURLConnection

The HttpURLConnection class extends URLConnection and provides additional methods for working with HTTP connections.

public int getResponseCode(): Returns the response code of the connection.
public String getResponseMessage(): Returns the response message of the connection.
public void setRequestMethod(String method): Sets the request method for the connection.
public String getRequestMethod(): Returns the request method for the connection.
public void setRequestProperty(String key, String value): Sets a request property on the connection.
public void addRequestProperty(String key, String value): Adds a request property to the connection.
public String getRequestProperty(String key): Returns the value of the specified request property.
public Map<String, List<String>> getRequestProperties(): Returns a map of the request properties.

The URLStreamHandler Class

This class is used to create a custom protocol handler for URLs.

protected URLConnection openConnection(URL u): Opens a connection to the specified URL.
protected void parseURL(URL u, String spec, int start, int limit): Parses the URL and sets the fields of the URL object.
protected boolean sameFile(URL u1, URL u2): Compares two URLs to see if they refer to the same file.
protected int getDefaultPort(): Returns the default port for the protocol.
protected InetAddress getHostAddress(URL u): Returns the IP address of the host.
protected boolean hostsEqual(URL u1, URL u2): Compares the hosts of two URLs.
protected int hashCode(URL u): Returns the hash code for the URL.
protected boolean equals(URL u1, URL u2): Compares two URLs for equality.
protected String toExternalForm(URL u): Returns the external form of the URL.
protected void setURL(URL u, String protocol, String host, int port, String authority, String userInfo, String path, String query, String ref): Sets the fields of the URL object.
protected void setURL(URL u, String protocol, String host, int port, String file, String ref): Sets the fields of the URL object.
protected String toExternalForm(URL u): Returns the external form of the URL.
protected void setURL(URL u, String protocol, String host, int port, String file, String ref): Sets the fields of the URL object.

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