Java try-with-resources statement vs. try-finally block

The try-with-resources is a feature added in Java 1.7. A resource is an object implementing the java.lang.AutoCloseable interface. The try-with-resources statement declares a block on one or more resources. The resources automatically close at the end of the block. The resources are always closed regardless of exceptions. If more than one resources are included, the resources are separated by semicolon.

1. An example of using the try-with-resources block.

public void printData(String sql) throws SQLException {
      //try-with-resources block
      try (PreparedStatement ps = con.prepareStatement(sql)) {
            //try-with-resources block
            try (ResultSet rs = ps.executeQuery()) {
                  //code processing the ResultSet.
                  while (rs.next()) {
                        System.out.println(rs.getString(1));
                  }
            }
      }
}

The above code automatically closes the rs at the end of the inner try-with-resources block and the ps at the end of the outer try-with-resources block.

2. Comparing the above try-with-resources code with the below try-finally code.

public void printData(String sql) throws SQLException {
      PreparedStatement ps = null;
      ResultSet rs = null;

      //try-finally block
      try {
            ps = con.prepareStatement(sql);
            rs = ps.executeQuery();
            //code processing the ResultSet.
            while (rs.next()) {
                 System.out.println(rs.getString(1));
            }
      } finally {
            if (rs != null) rs.close();
            if (ps != null) ps.close();
      }
}

Both codes ensure that the rs and ps are closed. However, if the database query code or the result processing code throws a SQLException and then one of the rs.close() and ps.close() methods also throws a SQLException, the SQLException thrown by the printData(String sql) is different using try-with-resources statement from try-finally statement. With the try-with-resources statement, the printData(String sql) throws the SQLException occurred from the database query code or the result processing code. Whereas the printData(String sql) throws the SQLException generated by the close() method if the  try-finally block is used.

3. Examples of including more than one resources.

try (BufferedReader reader = new BufferedReader(new FileReader(file));
             BufferedWriter writer = new BufferedWriter(new FileWriter(file2, true))) {
      String line;
      while (line = reader.readLine() != null) {
            writer.write(line);
      }
}

public void printData(String sql) throws SQLException {    
      try (PreparedStatement ps = con.prepareStatement(sql); ResultSet rs = ps.executeQuery()) {
            //code processing the ResultSet.
            while (rs.next()) {
                  System.out.println(rs.getString(1));
            }
      }

}
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