CODEWITHSUNDEEP
javamysqleclipse
Glyn
Glyn

Reputation: 2005

Correct way to catch an exception in Java after reading a DB

I am using Eclipse Juno, Java and MySQL.

I am just cleaning up my code and would greatly appreciate any advice on the correct way to catch exceptions after reading the database.

When I read my MySQL DB with the following code:

public List<AccountAndCubs> getAccountAndCubs(String AccountId, String user, String pass, String level, String pack, Date archived, String acaId, String cdId, String surname, String firstname) {
    List<AccountAndCubs> accountAndCubsList = new ArrayList<AccountAndCubs>();
    AccountAndCubs accountAndCubs = null; // necessary unless you do something in the exception handler
    accountAndCubsList.clear();
    ResultSet result = null;
    PreparedStatement ps = null;


    try {
      ps = conn.prepareStatement(
      "SELECT at_accounts.*, at_account_cub_association.aca_id," +
              " at_cub_details.cd_id, at_cub_details.cd_surname, at_cub_details.cd_first_name" +
        " FROM at_accounts" + 
        " LEFT JOIN at_account_cub_association ON at_accounts.acc_id = at_account_cub_association.acc_id" +
        " LEFT JOIN at_cub_details ON at_account_cub_association.cd_id = at_cub_details.cd_id" +
        " WHERE (at_accounts.acc_email_address = \"" + user + "\"" + ")" +
        " ORDER BY at_cub_details.cd_surname, at_cub_details.cd_first_name;");

      result = ps.executeQuery();
      while (result.next()) {
          accountAndCubs = new AccountAndCubs(result.getString(1), result.getString(2), result.getString(3), result.getString(4), result.getString(5), result.getDate(6), result.getString(7), result.getString(8), result.getString(9), result.getString(10));
          accountAndCubsList.add(accountAndCubs);
      }
    }
    catch (SQLException e) {
      //do stuff on fail
        System.out.println("SQLException getAccountAndCubs 1.");
        e.printStackTrace();
    }
    finally {
        if (result != null) {
            try {
                result.close();
            }
            catch (SQLException e) {
                System.out.println("SQLException getAccountAndCubs 2.");
                e.printStackTrace();
            }
        }
        if (ps != null) {
            try {
                ps.close();
            }   
            catch (SQLException e) {
                System.out.println("SQLException getAccountAndCubs 3.");
                e.printStackTrace();
            }
        }
    }
    return accountAndCubsList;
}

I get the following error in the "Development Mode" panel:

Uncaught exception escaped.
11:47:18.780 [ERROR] [org.AwardTracker.AwardTracker] Uncaught exception escaped

java.lang.NullPointerException: null
at org.AwardTracker.client.AccountUpdateView.renderAccountAndCubsTable(AccountUpdateView.java:444)
at org.AwardTracker.client.AccountUpdateView$GetAccountAndCubsHandler.onSuccess(AccountUpdateView.java:390)
at org.AwardTracker.client.AccountUpdateView$GetAccountAndCubsHandler.onSuccess(AccountUpdateView.java:1)
at com.google.gwt.user.client.rpc.impl.RequestCallbackAdapter.onResponseReceived(RequestCallbackAdapter.java:232)
at com.google.gwt.http.client.Request.fireOnResponseReceived(Request.java:287)
at com.google.gwt.http.client.RequestBuilder$1.onReadyStateChange(RequestBuilder.java:395)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
at com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:338)
at com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:219)
at com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:136)
at com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:571)
at com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:279)
at com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:91)
at com.google.gwt.core.client.impl.Impl.apply(Impl.java)
at com.google.gwt.core.client.impl.Impl.entry0(Impl.java:242)
at sun.reflect.GeneratedMethodAccessor24.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
at com.google.gwt.dev.shell.BrowserChannelServer.reactToMessages(BrowserChannelServer.java:293)
at com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:547)
at com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:364)
at java.lang.Thread.run(Unknown Source)

The result is returned to the client side, as expected, with a row from the "at_accounts" table and no result from the other tables. So no error is resulting on the client side. This is just a clean up so I can write this code correctly and catch the exception before it does become a problem.

Your advice on the correct way to catch exception after reading the database would be greatly appreciated.

Regards,

Glyn

Upvotes: 0

Views: 190

Answers (3)

Gerard Cristofol
Gerard Cristofol

Reputation: 46

In addition to the comments above you might use the new try-with-resources, it ensures that each resource is closed at the end of the statement thus making your code simpler.

See http://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

Upvotes: 2

James P.
James P.

Reputation: 19617

What you can do is wrap the Exception in a custom-made one suited to higher levels, say DataLayerException. Then rethrow to let it bubble up. Same applies for the service layer (DataLayerException is wrapped in a ServiceLayerException). When the Exception reaches the UI you can intercept it and produce a meaningful message.

Here are some links for reference:

Upvotes: 1

Zim-Zam O&#39;Pootertoot
Zim-Zam O&#39;Pootertoot

Reputation: 18158

A NullPointerException almost always indicates an error condition that shouldn't be caught except to be logged or re-thrown (i.e. you shouldn't just ignore the exception). Determine which object is causing the exception, and then determine if it makes sense for the object to be null; if it makes sense for the object to be null then precede the statement throwing the exception with a if(object != null) guard, and if it doesn't make sense for the object to be null then fix the erroneous code that is setting the object to null.

Upvotes: 1

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