Handling SQLExceptions
When JDBC encounters an error during an interaction with a data source, it throws an instance of SQLException as opposed to Exception . (A data source in this context represents the database to which a Connection object is connected.) The SQLException instance contains the following information that can help you determine the cause of the error:
- A description of the error. Retrieve the String object that contains this description by calling the method SQLException.getMessage .
- A SQLState code. These codes and their respective meanings have been standardized by ISO/ANSI and Open Group (X/Open), although some codes have been reserved for database vendors to define for themselves. This String object consists of five alphanumeric characters. Retrieve this code by calling the method SQLException.getSQLState .
- An error code. This is an integer value identifying the error that caused the SQLException instance to be thrown. Its value and meaning are implementation-specific and might be the actual error code returned by the underlying data source. Retrieve the error by calling the method SQLException.getErrorCode .
- A cause. A SQLException instance might have a causal relationship, which consists of one or more Throwable objects that caused the SQLException instance to be thrown. To navigate this chain of causes, recursively call the method SQLException.getCause until a null value is returned.
- A reference to any chained exceptions. If more than one error occurs, the exceptions are referenced through this chain. Retrieve these exceptions by calling the method SQLException.getNextException on the exception that was thrown.
Retrieving Exceptions
The following method, JDBCTutorialUtilities.printSQLException , outputs the SQLState, error code, error description, and cause (if there is one) contained in the SQLException as well as any other exception chained to it:
public static void printSQLException(SQLException ex) < for (Throwable e : ex) < if (e instanceof SQLException) < if (ignoreSQLException( ((SQLException)e). getSQLState()) == false) < e.printStackTrace(System.err); System.err.println("SQLState: " + ((SQLException)e).getSQLState()); System.err.println("Error Code: " + ((SQLException)e).getErrorCode()); System.err.println("Message: " + e.getMessage()); Throwable t = ex.getCause(); while(t != null) < System.out.println("Cause: " + t); t = t.getCause(); >> > > > For example, if you call the method CoffeesTable.dropTable with Java DB as your DBMS, the table COFFEES does not exist, and you remove the call to JDBCTutorialUtilities.ignoreSQLException , the output will be similar to the following:
SQLState: 42Y55 Error Code: 30000 Message: 'DROP TABLE' cannot be performed on 'TESTDB.COFFEES' because it does not exist.
Instead of printing SQLException information, you could instead first retrieve the SQLState then process the SQLException accordingly. For example, the method JDBCTutorialUtilities.ignoreSQLException returns true if the SQLState is equal to code 42Y55 (and you are using Java DB as your DBMS), which causes JDBCTutorialUtilities.printSQLException to ignore the SQLException :
public static boolean ignoreSQLException(String sqlState) < if (sqlState == null) < System.out.println("The SQL state is not defined!"); return false; >// X0Y32: Jar file already exists in schema if (sqlState.equalsIgnoreCase("X0Y32")) return true; // 42Y55: Table already exists in schema if (sqlState.equalsIgnoreCase("42Y55")) return true; return false; > Retrieving Warnings
SQLWarning objects are a subclass of SQLException that deal with database access warnings. Warnings do not stop the execution of an application, as exceptions do; they simply alert the user that something did not happen as planned. For example, a warning might let you know that a privilege you attempted to revoke was not revoked. Or a warning might tell you that an error occurred during a requested disconnection.
A warning can be reported on a Connection object, a Statement object (including PreparedStatement and CallableStatement objects), or a ResultSet object. Each of these classes has a getWarnings method, which you must invoke in order to see the first warning reported on the calling object. If getWarnings returns a warning, you can call the SQLWarning method getNextWarning on it to get any additional warnings. Executing a statement automatically clears the warnings from a previous statement, so they do not build up. This means, however, that if you want to retrieve warnings reported on a statement, you must do so before you execute another statement.
The following methods from JDBCTutorialUtilities.java illustrate how to get complete information about any warnings reported on Statement or ResultSet objects:
public static void getWarningsFromResultSet(ResultSet rs) throws SQLException < JDBCTutorialUtilities.printWarnings(rs.getWarnings()); >public static void getWarningsFromStatement(Statement stmt) throws SQLException < JDBCTutorialUtilities.printWarnings(stmt.getWarnings()); >public static void printWarnings(SQLWarning warning) throws SQLException < if (warning != null) < System.out.println("\n---Warning---\n"); while (warning != null) < System.out.println("Message: " + warning.getMessage()); System.out.println("SQLState: " + warning.getSQLState()); System.out.print("Vendor error code: "); System.out.println(warning.getErrorCode()); System.out.println(""); warning = warning.getNextWarning(); >> The most common warning is a DataTruncation warning, a subclass of SQLWarning . All DataTruncation objects have a SQLState of 01004 , indicating that there was a problem with reading or writing data. DataTruncation methods let you find out in which column or parameter data was truncated, whether the truncation was on a read or write operation, how many bytes should have been transferred, and how many bytes were actually transferred.
Categorized SQLExceptions
Your JDBC driver might throw a subclass of SQLException that corresponds to a common SQLState or a common error state that is not associated with a specific SQLState class value. This enables you to write more portable error-handling code. These exceptions are subclasses of one of the following classes:
See the latest Javadoc of the java.sql package or the documentation of your JDBC driver for more information about these subclasses.
Other Subclasses of SQLException
The following subclasses of SQLException can also be thrown:
- BatchUpdateException is thrown when an error occurs during a batch update operation. In addition to the information provided by SQLException , BatchUpdateException provides the update counts for all statements that were executed before the error occurred.
- SQLClientInfoException is thrown when one or more client information properties could not be set on a Connection. In addition to the information provided by SQLException , SQLClientInfoException provides a list of client information properties that were not set.
JDBC — Exceptions Handling
Exception handling allows you to handle exceptional conditions such as program-defined errors in a controlled fashion.
When an exception condition occurs, an exception is thrown. The term thrown means that current program execution stops, and the control is redirected to the nearest applicable catch clause. If no applicable catch clause exists, then the program’s execution ends.
JDBC Exception handling is very similar to the Java Exception handling but for JDBC, the most common exception you’ll deal with is java.sql.SQLException.
SQLException Methods
An SQLException can occur both in the driver and the database. When such an exception occurs, an object of type SQLException will be passed to the catch clause.
The passed SQLException object has the following methods available for retrieving additional information about the exception −
| Method | Description |
|---|---|
| getErrorCode( ) | Gets the error number associated with the exception. |
| getMessage( ) | Gets the JDBC driver’s error message for an error, handled by the driver or gets the Oracle error number and message for a database error. |
| getSQLState( ) | Gets the XOPEN SQLstate string. For a JDBC driver error, no useful information is returned from this method. For a database error, the five-digit XOPEN SQLstate code is returned. This method can return null. |
| getNextException( ) | Gets the next Exception object in the exception chain. |
| printStackTrace( ) | Prints the current exception, or throwable, and it’s backtrace to a standard error stream. |
| printStackTrace(PrintStream s) | Prints this throwable and its backtrace to the print stream you specify. |
| printStackTrace(PrintWriter w) | Prints this throwable and it’s backtrace to the print writer you specify. |
By utilizing the information available from the Exception object, you can catch an exception and continue your program appropriately. Here is the general form of a try block −
try < // Your risky code goes between these curly braces. >catch(Exception ex) < // Your exception handling code goes between these // curly braces, similar to the exception clause // in a PL/SQL block. >finally < // Your must-always-be-executed code goes between these // curly braces. Like closing database connection. >
Example
Study the following example code to understand the usage of try. catch. finally blocks.
import java.sql.CallableStatement; import java.sql.Connection; import java.sql.DriverManager; import java.sql.SQLException; public class JDBCExample < static final String DB_URL = "jdbc:mysql://localhost/TUTORIALSPOINT"; static final String USER = "guest"; static final String PASS = "guest123"; static final String QUERY = ""; public static void main(String[] args) < // Open a connection try(Connection conn = DriverManager.getConnection(DB_URL, USER, PASS); CallableStatement stmt = conn.prepareCall(QUERY); ) < // Bind values into the parameters. stmt.setInt(1, 1); // This would set ID // Because second parameter is OUT so register it stmt.registerOutParameter(2, java.sql.Types.VARCHAR); //Use execute method to run stored procedure. System.out.println("Executing stored procedure. " ); stmt.execute(); //Retrieve employee name with getXXX method String empName = stmt.getString(2); System.out.println("Emp Name with ID: 1 is " + empName); >catch (SQLException e) < e.printStackTrace(); >> > Now, let us compile the above example as follows −
When you run JDBCExample, it produces the following result if there is no problem, otherwise the corresponding error would be caught and error message would be displayed −
C:\>java JDBCExample Executing stored procedure. Emp Name with ID: 1 is Zara C:\>
Try the above example by passing wrong database name or wrong username or password and check the result.