Client 2 - Adding Error Checking

Client 2 ”Adding Error Checking

In the previous section, I mentioned that the DriverManager.getConnection() method will throw an exception whenever it fails. Listing 13.3 shows the second JDBC client. This version is nearly identical to client1.java, except that in client1 , you ignored any exceptions and in client2 , you will intercept them and produce friendlier error messages.

Listing 13.3 client2.java

 1 //  2 // File: client2.java  3 //  4  5 import java.sql.*;  6  7 public class client2  8 {  9   public static void main( String args[] ) 10   { 11     String  driver = "org.postgresql.Driver"; 12     String  url    = "jdbc:postgresql:movies"; 13     String  user   = "korry"; 14     String  pwd    = "cows"; 15 16     try 17     { 18       Class.forName( driver ); 19     } 20     catch( ClassNotFoundException e ) 21     { 22       System.err.println( "Can't load driver" + e.getMessage()); 23       System.exit( 1 ); 24     } 25 26     try 27     { 28       Connection con = DriverManager.getConnection(url, user, pwd); 29 30       System.out.println( "Connection attempt successful" ); 31 32       con.close(); 33 34     } 35     catch( Exception e ) 36     { 37       System.err.println( "Connection attempt failed" ); 38       System.err.println( e.getMessage()); 39     } 40   } 41 } 

The first difference between client1 and client2 appears at line 10. In the client1 version, you had to declare that main() could throw ClassNotFoundException and SQLException . You'll be intercepting those exceptions now, so main() should not throw any exceptions.

At lines 17 through 24, you wrap the call to Class.forName() in a try/catch block. Remember that forName() dynamically loads the implementation of a class into your VM ”it is entirely possible that forName() may not be able to load the class file that you need. You may have misspelled the class name , or the class file might not be in your $CLASSPATH search path . You could also find that you don't have the permissions required to load the class file, or you could even find that the class file has been corrupted. If you catch an exception, print a suitable error message and exit.

After the Driver class has been loaded into your VM, you can attempt to make a connection. Wrap the connection attempt in a try/catch block so that you can intercept any exceptions. The call to DriverManager.getConnection() throws a SQLException if something goes wrong. Let's compile this application and give it a try:

 $ make client2.class javac -g client2.java $ java client2 Connection attempt failed Connection refused. Check that the hostname and port is correct, and that the postmaster is running with the -i flag, which enables TCP/IP networking. $ 

I haven't restarted the postmaster yet, so I encounter the same error as before, but this time the error message is less intimidating.

DriverManager.getConnection() can throw two kinds of exceptions: SQLException and PSQLException ( PSQLException is derived from SQLException ). PSQLException s are specific to the PostgreSQL driver; SQLException s indicate errors that might be common to many drivers. Let's modify client2.java so that you can see which type of exception you catch. The new client is shown in Listing 13.4.

Listing 13.4 client2a.java

 1 //  2 // File: client2a.java  3 //  4  5 import java.sql.*;  6 import org.postgresql.util.PSQLException;  7  8 public class client2a  9 { 10   public static void main( String args[] ) 11   { 12     String  driver = "org.postgresql.Driver"; 13 14     try 15     { 16       Class.forName( driver ); 17     } 18     catch( ClassNotFoundException e ) 19     { 20       System.err.println( "Can't load driver " + e.getMessage()); 21       System.exit( 1 ); 22     } 23     catch( Exception e ) 24     { 25       System.err.println( "Can't load driver " + e.toString()); 26       System.exit( 1 ); 27     } 28 29     try 30     { 31       Connection con = DriverManager.getConnection( args[0] ); 32 33       System.out.println( "Connection attempt successful" ); 34 35       con.close(); 36 37     } 38     catch( PSQLException e) 39     { 40       System.err.println( "Connection failed(PSQLException)" ); 41       System.err.println( e.getMessage()); 42     } 43     catch( SQLException e ) 44     { 45       System.err.println( "Connection failed(SQLException)" ); 46       System.err.println( e.getMessage()); 47     } 48   } 49 } 

I've made a few minor changes in client2a.java . You want to distinguish between SQLException and PSQLException , so at line 6, import the appropriate package. I've also removed most of the String variables used in the previous version. In this version, you supply a URL on the command line rather than hard-coding the connection parameters. At line 31, you call a different flavor of the getConnection() method; this one expects a single argument (the URL to which you want to connect). Notice that I have removed the hard-coded URL from this client. When you invoke client2a , you provide a connection URL on the command line (see the next example). Finally, you will catch PSQLException explicitly.

Compile this client and reproduce the same error that you saw earlier:

 $ make client2a.class javac -g client2a.java $ java client2a "jdbc:postgresql:movies?user=korry&password=cows" Connection failed(PSQLException) Connection refused. Check that the hostname and port is correct, and that the postmaster is running with the -i flag, which enables TCP/IP networking. $ 

Okay, that message comes from a PSQLException . Now, let's restart the postmaster and try connecting with an invalid password:

 $ pg_ctl start -l /tmp/pg.log -o -i postmaster successfully started $ java client2a "jdbc:postgresql:movies?user=korry&password=oxen" Connection failed(PSQLException) Something unusual has occurred to cause the driver to fail. Please report this exception: Exception: java.sql.SQLException:   FATAL 1:  Password authentication failed for user "korry" Stack Trace: java.sql.SQLException: FATAL 1:  Password authentication failed for user "korry"         at org.postgresql.Connection.openConnection(Unknown Source)         at org.postgresql.Driver.connect(Unknown Source)         at java.sql.DriverManager.getConnection(DriverManager.java:517)         at java.sql.DriverManager.getConnection(DriverManager.java:199)         at client2a.main(client2a.java:31) End of Stack Trace 

We're back to the intimidating error messages again. This is still a PSQLException , but the PostgreSQL JDBC Driver feels that an invalid password is unusual enough to justify this kind of error. You can see the importance of catching exceptions ”you may want to translate this sort of message into something a little less enthusiastic rather than attacking your users with the raw error message text, as we've done here.

It's a little harder to generate a SQLException when the only thing you are doing is connecting and disconnecting. If you try hard enough, you can break just about anything:

 $ java client2a "jdbc:postgres:movies?user=korry&password=cows" Connection failed(SQLException) No suitable driver 

In this example, I've misspelled the subprotocol portion of the connection URL ( postgres should be postgresql ).

Now, let's move on to the next topic: command processing.