The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
A DATALINK
value references a resource outside the underlying data source through a URL. A URL, uniform resource locator, is a pointer to a resource on the World Wide Web. A resource can be something as simple as a file or a directory, or it can be a reference to a more complicated object, such as a query to a database or to a search engine.
The following topics are covered:
Use the method PreparedStatement.setURL
to specify a java.net.URL
object to a prepared statement. In cases where the type of URL being set is not supported by the Java platform, store the URL with the setString
method.
For example, suppose the owner of The Coffee Break would like to store a list of important URLs in a database table. The following method,
DatalinkSample.addURLRow
, adds one row of data to the table DATA_REPOSITORY
. The row consists of a string identifying the URL, DOCUMENT_NAME
and the URL itself, URL
:
public void addURLRow(String description, String url) throws SQLException { String query = "INSERT INTO data_repository(document_name,url) VALUES (?,?)"; try (PreparedStatement pstmt = this.con.prepareStatement(query)) { pstmt.setString(1, description); pstmt.setURL(2,new URL(url)); pstmt.execute(); } catch (SQLException sqlex) { JDBCTutorialUtilities.printSQLException(sqlex); } catch (Exception ex) { System.out.println("Unexpected exception"); ex.printStackTrace(); } }
Use the method ResultSet.getURL
to retrieve a reference to external data as a java.net.URL
object. In cases where the type of URL returned by the methods getObject
or getURL
is not supported by the Java platform, retrieve the URL as a String
object by calling the method getString
.
The following method,
DatalinkSample.viewTable
, displays the contents of all the URLs stored in the table DATA_REPOSITORY
:
public static void viewTable(Connection con, Proxy proxy) throws SQLException, IOException { String query = "SELECT document_name, url FROM data_repository"; try (Statement stmt = con.createStatement()) { ResultSet rs = stmt.executeQuery(query); if ( rs.next() ) { String documentName = null; java.net.URL url = null; documentName = rs.getString(1); // Retrieve the value as a URL object. url = rs.getURL(2); if (url != null) { // Retrieve the contents from the URL. URLConnection myURLConnection = url.openConnection(proxy); BufferedReader bReader = new BufferedReader(new InputStreamReader(myURLConnection.getInputStream())); System.out.println("Document name: " + documentName); String pageContent = null; while ((pageContent = bReader.readLine()) != null ) { // Print the URL contents System.out.println(pageContent); } } else { System.out.println("URL is null"); } } } catch (SQLException e) { JDBCTutorialUtilities.printSQLException(e); } catch(IOException ioEx) { System.out.println("IOException caught: " + ioEx.toString()); } catch (Exception ex) { System.out.println("Unexpected exception"); ex.printStackTrace(); } }
The sample
DatalinkSample.java
stores the Oracle URL, https://www.oracle.com in the table DATA_REPOSITORY
. Afterward, it displays the contents of all documents referred to by the URLs stored in DATA_REPOSITORY
, which includes the Oracle home page, https://www.oracle.com.
The sample retrieves the URL from the result set as a java.net.URL
object with the following statement:
url = rs.getURL(2);
The sample accesses the data referred to by the URL
object with the following statements:
// Retrieve the contents from the URL. URLConnection myURLConnection = url.openConnection(proxy); BufferedReader bReader = new BufferedReader(new InputStreamReader(myURLConnection.getInputStream())); System.out.println("Document name: " + documentName); String pageContent = null; while ((pageContent = bReader.readLine()) != null ) { // Print the URL contents System.out.println(pageContent); }
The method URLConnection.openConnection
can take no arguments, which means that the URLConnection
represents a direct connection to the Internet. If you require a proxy server to connect to the Internet, the openConnection
method accepts a java.net.Proxy
object as an argument. The following statements demonstrate how to create an HTTP proxy with the server name www-proxy.example.com
and port number 80
:
Proxy myProxy; InetSocketAddress myProxyServer; myProxyServer = new InetSocketAddress("www-proxy.example.com", 80); myProxy = new Proxy(Proxy.Type.HTTP, myProxyServer);