Spec-Zone .ru
спецификации, руководства, описания, API
|
|
Spec-Zone .ru
спецификации, руководства, описания, API
|
NOTE: The material in this chapter is based on JDBCtm API Tutorial and Reference, Second Edition: Universal Data Access for the Javatm 2 Platform, published by Addison Wesley as part of the Java series, ISBN 0-201-43328-1.
A CallableStatement object provides a way to call stored procedures in a standard way for all RDBMSs. A stored procedure is stored in a database; the call to the stored procedure is what a CallableStatement object contains. This call is written in an escape syntax that may take one of two forms: one form with a result parameter, and the other without one. A result parameter, a kind of OUT parameter, is the return value for the stored procedure. Both forms may have a variable number of parameters used for input (IN parameters), output (OUT parameters), or both (INOUT parameters). A question mark serves as a placeholder for a parameter.
The syntax for invoking a stored procedure using the JDBC API is shown here. Note that the square brackets indicate that what is between them is optional; they are not themselves part of the syntax.
{call procedure_name[(?, ?, ...)]}
The syntax for a procedure that returns a result parameter is:
{? = call procedure_name[(?, ?, ...)]}
The syntax for a stored procedure with no parameters would look like this:
{call procedure_name}
Normally, anyone creating a CallableStatement object would already know that the DBMS being used supports stored procedures and what those procedures are. If one needed to check, however, various DatabaseMetaData methods will supply such information. For instance, the method supportsStoredProcedures will return true if the DBMS supports stored procedure calls, and the method getProcedures will return a description of the stored procedures available.
CallableStatement inherits Statement methods, which deal with SQL statements in general, and it also inherits PreparedStatement methods, which deal with IN parameters. All of the methods defined in CallableStatement deal with OUT parameters or the output aspect of INOUT parameters: registering the JDBC types of the OUT parameters, retrieving values from them, or checking whether a returned value was JDBC NULL. Whereas the getXXX methods defined in ResultSet retrieve values from a result set, the getXXX methods in CallableStatement retrieve values from the OUT parameters and/or return value of a stored procedure.
CallableStatement objects are created with the Connection method prepareCall. The following example, in which con is an active JDBC Connection object, creates an instance of CallableStatement.
CallableStatement cstmt = con.prepareCall(
"{call getTestData(?, ?)}");
The variable cstmt contains a call to the stored procedure getTestData, which has two input parameters and no result parameter. Whether the ? placeholders are IN, OUT, or INOUT parameters depends on the stored procedure getTestData. This instance of a CallableStatement object was created using JDBC 1.0 API; consequently, any query in the stored procedure called by cstmt will produce a default ResultSet object (one that is non-scrollable and non-updatable).
The JDBC 2.0 API provides the means to create CallableStatement objects that can produce ResultSet objects that are scrollable and updatable, as the following code fragment demonstrates.
String sql = "{call getTestData(?, ?)}";
CallableStatement cstmt2 = con.prepareCall(sql,
ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);
The variable cstmt2 contains the same call to the stored procedure TestData that cstmt does, but with cstmt2, any resultSet objects that TestData produces can be updated and are scrollable (though they will not be sensitive to updates made while they are open). Explanations for the constants used to indicate scrollability and updatability are given in the chapter "ResultSet."
Passing in any IN parameter values to a CallableStatement object is done using the setXXX methods inherited from PreparedStatement. The type of the value being passed in determines which setXXX method to use (setFloat to pass in a float value, setBoolean to pass in a boolean, and so on). Of the programs that use parameters, the vast majority use only IN parameters.
The ability to make batch updates is the same for CallableStatement objects as it is for PreparedStatement objects. In fact, a CallableStatement object is restricted to the same functionality that a PreparedStatement object has. More precisely, when using the batch update facility, a CallableStatement object can call only stored procedures that take input parameters or no parameters at all. Further, the stored procedure must return an update count. The CallableStatement.executeBatch method (inherited from PreparedStatement) will throw a BatchUpdateException if the stored procedure returns anything other than an update count or takes OUT or INOUT parameters.
The following code fragment illustrates using the batch update facility to associate two sets of parameters with a CallableStatement object.
CallableStatement cstmt = con.prepareCall(
"{call updatePrices(?, ?)}");
cstmt.setString(1, "Colombian");
cstmt.setFloat(2, 8.49f);
cstmt.addBatch();
cstmt.setString(1, "Colombian_Decaf");
cstmt.setFloat(2, 9.49f);
cstmt.addBatch();
int [] updateCounts = cstmt.executeBatch();
The variable cstmt contains a call to the stored procedure updatePrices with two sets of parameters associated with it. When cstmt is executed, two update statements will be executed together as a batch: one with the parameters Colombian and 8.49f, and a second one with the parameters Colombian_Decaf and 9.49f. An f after a number, as in 8.49f, tells the Java compiler that the value is a float; otherwise, the compiler assumes that a number with decimal digits is a double and will not allow it to be used as a float.
If the stored procedure returns OUT parameters, the JDBC type of each OUT parameter must be registered before the CallableStatement object can be executed. This is necessary because some DBMSs require the SQL type (which the JDBC type represents), not because JDBC requires it. JDBC types, a set of generic SQL type identifiers that represent the most commonly used SQL types, are explained fully in the chapter "Mapping SQL and Java Types" on page 85.
Registering the JDBC type is done with the method registerOutParameter. Then after the statement has been executed, CallableStatement's getXXX methods can be used to retrieve OUT parameter values. The correct CallableStatement.getXXX method to use is the type in the Java programming language that corresponds to the JDBC type registered for that parameter. (The standard mapping from JDBC types to Java types is shown in Table 8.1 on page 105.) In other words, registerOutParameter uses a JDBC type (so that it matches the data type that the database will return), and getXXX casts this to a Java type.
To illustrate, the following code registers the OUT parameters, executes the stored procedure called by cstmt, and then retrieves the values returned in the OUT parameters. The method getByte retrieves a Java byte from the first OUT parameter, and getBigDecimal retrieves a java.math.BigDecimal object (with three digits after the decimal point) from the second OUT parameter. The method executeQuery is used to execute cstmt because the stored procedure that it calls returns a result set.
CallableStatement cstmt = con.prepareCall(
"{call getTestData(?, ?)}");
cstmt.registerOutParameter(1, java.sql.Types.TINYINT);
cstmt.registerOutParameter(2, java.sql.Types.DECIMAL, 3);
ResultSet rs = cstmt.executeQuery();
// . . . retrieve result set values with rs.getXXX methods
byte x = cstmt.getByte(1);
java.math.BigDecimal n = cstmt.getBigDecimal(2);
Unlike ResultSet, CallableStatement does not provide a special mechanism for retrieving large OUT values incrementally. More specifically, it does not have getXXX methods for streams of data, such as getAsciiStream or getBinary-Stream. However, the JDBC 2.0 API provides CallableStatement methods for retrieving SQL3 datatypes as OUT or INOUT parameters, which includes the methods getBlob and getClob for retrieving binary large objects and character large objects.
When a method takes an int specifying which parameter to act upon (setXXX, getXXX, and registerOutParameter), that int refers to ? placeholder parameters only, with numbering starting at one. The parameter number does not refer to literal parameters that might be supplied to a stored procedure call. For example, the following code fragment illustrates a stored procedure call with one literal parameter and one ? parameter:
CallableStatement cstmt = con.prepareCall(
"{call getTestData(25, ?)}");
cstmt.registerOutParameter(1, java.sql.Types.TINYINT);
In this code, the first argument to registerOutParameter, the int 1, refers to the first ? parameter (and in this case, the only ? parameter). It does not refer to the literal 25, which is the first parameter to the stored procedure.
A parameter that supplies input as well as accepts output (an INOUT parameter) requires a call to the appropriate setXXX method (inherited from PreparedStatement) in addition to a call to the method registerOutParameter. The setXXX method sets a parameter's value as an input parameter, and the method registerOutParameter registers its JDBC type as an output parameter. The setXXX method provides a Java value that the driver converts to a JDBC value before sending it to the database. The JDBC type of this IN value and the JDBC type supplied to the method registerOutParameter should be the same. Then, to retrieve the output value, a corresponding getXXX method is used. For example, a parameter whose Java type is byte should use the method setByte to assign the input value, should supply a TINYINT as the JDBC type to registerOutParameter, and should use getByte to retrieve the output value. ("Mapping SQL and Java Types" on page 85 contains tables of type mappings.)
The following example assumes that there is a stored procedure reviseTotal whose only parameter is an INOUT parameter. The method setByte sets the parameter to 25, which the driver will send to the database as a JDBC TINYINT. Next registerOutParameter registers the parameter as a JDBC TINYINT. After the stored procedure is executed, a new JDBC TINYINT value is returned, and the method getByte will retrieve this new value as a Java byte. Since the stored procedure called in this example returns an update count, the method executeUpdate is used.
CallableStatement cstmt = con.prepareCall(
"{call reviseTotal(?)}");
cstmt.setByte(1, (byte)25);
cstmt.registerOutParameter(1, java.sql.Types.TINYINT);
cstmt.executeUpdate();
byte x = cstmt.getByte(1);
Because of limitations imposed by some DBMSs, it is recommended that for maximum portability, all of the results in a ResultSet object generated by the execution of a CallableStatement object should be retrieved before OUT parameters are retrieved. When all values have been retrieved from a result set, the method ResultSet.next will return false.
If a CallableStatement object returns multiple ResultSet objects (which is possible only if it is executed with a call to the method execute), all of the results should be retrieved before OUT parameters are retrieved. In this case, to be sure that all results have been accessed, the Statement methods getResultSet, getUpdateCount, and getMoreResults need to be called until there are no more results. When all results have been exhausted, the method getMoreResults returns false, and the method getUpdateCount returns -1.
After all values have been retrieved from ResultSet objects (using ResultSet.getXXX methods), and after it has been determined that there are no more update counts, values from OUT parameters can be retrieved (using CallableStatement.getXXX methods).
The value returned to an OUT parameter may be JDBC NULL. When this happens, the JDBC NULL value will be converted so that the value returned by a getXXX method will be null, 0, or false, depending on the getXXX method type. As with ResultSet objects, the only way to know if a value of 0 or false was originally JDBC NULL is to test it with the method wasNull, which returns true if the last value read by a getXXX method was JDBC NULL, and false otherwise.