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Subject: Re: Eclipse doesn't like else
Date: Sat, 13 May 2017 21:04:52 -0400
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On 5/12/2017 1:44 PM, Eric Douglas wrote:
> Why must it give a warning on this else condition?
>
> import java.sql.Connection;
> import java.sql.DriverManager;
> import java.sql.PreparedStatement;
> import java.sql.SQLException;
>
> public class CompileMe {
>      public static void testMe() {
>           try {
>                Connection conn = DriverManager.getConnection("db_connect_string", "db_userid", "db_password");
>                String someCondition = "check me";
>                PreparedStatement myPrep;
>                myPrep = conn.prepareStatement("initial query");
>                myPrep.executeQuery();
>                myPrep.close();
>                if (someCondition.equals("one")) {
>                     myPrep = conn.prepareStatement("query something");
>                } else {
>                     myPrep = conn.prepareStatement("query something else");
>                }
>                myPrep.executeQuery();
>                myPrep.close();
>           } catch (SQLException e) {
>                e.printStackTrace();
>           }
>      }
> }
>
> This example will run the same whether we have 2 calls to
> prepareStatement, or set one common string to pass into prepareStatement
> after the if, but if we write it this way Eclipse gives a warning
> "Resource leak: 'myPrep' is not closed at this location".
>
> Take out the 3 lines above the if (prepare, execute, and close a
 > different query) and the warning goes away.

As already explained by other then there are a resource leak.

And as I said in another reply then I don't think Eclipse detects
all resource leaks.

But if we take a step back and look at the code then there are
several things that comes to my mind:
* I do not like the reuse of the PreparedStatement ref I would
   use different ref's
* all statements and connection need to be closed to avoid resource
   leak (well - statements should be closed when connection is
   closed per specs, but I would not rely on that)
* the only way to get this right is to have a systematic
   approach

Here are 3 different approaches:

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.SQLException;

public class ToCloseOrNotToclose {
     // Approach 1:
     // * close everything as soon as possible in good flow
     // * close in catch block if necessary to handle bad flows
     public static void approach1(String cond) {
         Connection con = null;
         PreparedStatement upd1 = null;
         PreparedStatement upd2a = null;
         PreparedStatement upd2b = null;
         try {
              con = DriverManager.getConnection("db_connect_string", 
"db_userid", "db_password");
              upd1 = con.prepareStatement("initial query");
              upd1.executeQuery();
              upd1.close();
              if (cond.equals("one")) {
                  upd2a = con.prepareStatement("query something");
                  upd2a.executeQuery();
                  upd2a.close();
              } else {
                  upd2b = con.prepareStatement("query something else");
                  upd2b.executeQuery();
                  upd2b.close();
              }
              con.close();
         } catch (SQLException e) {
             e.printStackTrace();
             try {
                 if(upd1 != null && !upd1.isClosed()) upd1.close();
                 if(upd2a != null && !upd2a.isClosed()) upd2a.close();
                 if(upd2b != null && !upd2b.isClosed()) upd2b.close();
                 if(con != null && !con.isClosed()) con.close();
             } catch (SQLException e2) {
                 e2.printStackTrace();
                 // consider what to do here
             }
             // consider what to do here
         }
     }
     // Approach 2:
     // * close in finally block for both good and bad flows
     public static void approach2(String cond) {
         Connection con = null;
         PreparedStatement upd1 = null;
         PreparedStatement upd2a = null;
         PreparedStatement upd2b = null;
         try {
              con = DriverManager.getConnection("db_connect_string", 
"db_userid", "db_password");
              upd1 = con.prepareStatement("initial query");
              upd1.executeQuery();
              if (cond.equals("one")) {
                  upd2a = con.prepareStatement("query something");
                  upd2a.executeQuery();
              } else {
                  upd2b = con.prepareStatement("query something else");
                  upd2b.executeQuery();
              }
              con.close();
         } catch (SQLException e) {
             e.printStackTrace();
             // consider what to do here
         } finally {
             try {
                 if(upd1 != null) upd1.close();
                 if(upd2a != null) upd2a.close();
                 if(upd2b != null) upd2b.close();
                 if(con != null) con.close();
             } catch (SQLException e2) {
                 e2.printStackTrace();
                 // consider what to do here
             }
         }
     }
     // Approach 3:
     // * use Java 7+ try with resource
     public static void approach3(String cond) {
         try (Connection con = 
DriverManager.getConnection("db_connect_string", "db_userid", 
"db_password")) {
              try(PreparedStatement upd1 = con.prepareStatement("initial 
query")) {
                  upd1.executeQuery();
              }
              if (cond.equals("one")) {
                  try(PreparedStatement upd2a = 
con.prepareStatement("query something")) {
                      upd2a.executeQuery();
                  }
              } else {
                  try(PreparedStatement upd2b = 
con.prepareStatement("query something else")) {
                      upd2b.executeQuery();
                  }
              }
         } catch (SQLException e) {
             e.printStackTrace();
             // consider what to do here
         }
     }
}

Arne