使用 JDBC 处理 SQL 语句

In general, to process any SQL statement with JDBC, you follow these steps:

1. Establishing a connection.
2. Create a statement.
3. Execute the query.
4. Process the ResultSet object.
5. Close the connection.

This page uses the following method, CoffeesTables.viewTable, from the tutorial sample to demonstrate these steps. This method outputs the contents of the table COFFEES. This method will be discussed in more detail later in this tutorial:

public static void viewTable(Connection con, String dbName)
    throws SQLException {

    Statement stmt = null;
    String query = "select COF_NAME, SUP_ID, PRICE, " +
                   "SALES, TOTAL " +
                   "from " + dbName + ".COFFEES";
    try {
        stmt = con.createStatement();
        ResultSet rs = stmt.executeQuery(query);
        while (rs.next()) {
            String coffeeName = rs.getString("COF_NAME");
            int supplierID = rs.getInt("SUP_ID");
            float price = rs.getFloat("PRICE");
            int sales = rs.getInt("SALES");
            int total = rs.getInt("TOTAL");
            System.out.println(coffeeName + "\t" + supplierID +
                               "\t" + price + "\t" + sales +
                               "\t" + total);
        }
    } catch (SQLException e ) {
        JDBCTutorialUtilities.printSQLException(e);
    } finally {
        if (stmt != null) { stmt.close(); }
    }
}

Establishing Connections

First, establish a connection with the data source you want to use. A data source can be a DBMS, a legacy file system, or some other source of data with a corresponding JDBC driver. This connection is represented by a Connection object. See Establishing a Connection for more information.

Creating Statements

A Statement is an interface that represents a SQL statement. You execute Statement objects, and they generate ResultSet objects, which is a table of data representing a database result set. You need a Connection object to create a Statement object.

For example, CoffeesTables.viewTable creates a Statement object with the following code:

stmt = con.createStatement();

There are three different kinds of statements:

• Statement: Used to implement simple SQL statements with no parameters.
• PreparedStatement: (Extends Statement.) Used for precompiling SQL statements that might contain input parameters. See Using Prepared Statements for more information.
• CallableStatement: (Extends PreparedStatement.) Used to execute stored procedures that may contain both input and output parameters. See Stored Procedures for more information.

Executing Queries

To execute a query, call an execute method from Statement such as the following:

• execute: Returns true if the first object that the query returns is a ResultSet object. Use this method if the query could return one or more ResultSet objects. Retrieve the ResultSet objects returned from the query by repeatedly calling Statement.getResultSet.
• executeQuery: Returns one ResultSet object.
• executeUpdate: Returns an integer representing the number of rows affected by the SQL statement. Use this method if you are using INSERT, DELETE, or UPDATE SQL statements.

For example, CoffeesTables.viewTable executed a Statement object with the following code:

ResultSet rs = stmt.executeQuery(query);

See Retrieving and Modifying Values from Result Sets for more information.

Processing ResultSet Objects

You access the data in a ResultSet object through a cursor. Note that this cursor is not a database cursor. This cursor is a pointer that points to one row of data in the ResultSet object. Initially, the cursor is positioned before the first row. You call various methods defined in the ResultSet object to move the cursor.

For example, CoffeesTables.viewTable repeatedly calls the method ResultSet.next to move the cursor forward by one row. Every time it calls next, the method outputs the data in the row where the cursor is currently positioned:

try {
    stmt = con.createStatement();
    ResultSet rs = stmt.executeQuery(query);
    while (rs.next()) {
        String coffeeName = rs.getString("COF_NAME");
        int supplierID = rs.getInt("SUP_ID");
        float price = rs.getFloat("PRICE");
        int sales = rs.getInt("SALES");
        int total = rs.getInt("TOTAL");
        System.out.println(coffeeName + "\t" + supplierID +
                           "\t" + price + "\t" + sales +
                           "\t" + total);
    }
}
// ...

See Retrieving and Modifying Values from Result Sets for more information.

Closing Connections

When you are finished using a Statement, call the method Statement.close to immediately release the resources it is using. When you call this method, its ResultSet objects are closed.

For example, the method CoffeesTables.viewTable ensures that the Statement object is closed at the end of the method, regardless of any SQLException objects thrown, by wrapping it in a finally block:

} finally {
    if (stmt != null) { stmt.close(); }
}

JDBC throws an SQLException when it encounters an error during an interaction with a data source. See Handling SQL Exceptions for more information.

In JDBC 4.1, which is available in Java SE release 7 and later, you can use a try-with-resources statement to automatically close Connection, Statement, and ResultSet objects, regardless of whether an SQLException has been thrown. An automatic resource statement consists of a try statement and one or more declared resources. For example, you can modify CoffeesTables.viewTable so that its Statement object closes automatically, as follows:

public static void viewTable(Connection con) throws SQLException {

    String query = "select COF_NAME, SUP_ID, PRICE, " +
                   "SALES, TOTAL " +
                   "from COFFEES";

    try (Statement stmt = con.createStatement()) {

        ResultSet rs = stmt.executeQuery(query);

        while (rs.next()) {
            String coffeeName = rs.getString("COF_NAME");
            int supplierID = rs.getInt("SUP_ID");
            float price = rs.getFloat("PRICE");
            int sales = rs.getInt("SALES");
            int total = rs.getInt("TOTAL");
            System.out.println(coffeeName + ", " + supplierID +
                               ", " + price + ", " + sales +
                               ", " + total);
        }
    } catch (SQLException e) {
        JDBCTutorialUtilities.printSQLException(e);
    }
}

The following statement is an try-with-resources statement, which declares one resource, stmt, that will be automatically closed when the try block terminates:

try (Statement stmt = con.createStatement()) {
    // ...
}

See The try-with-resources Statement in the Essential Classes trail for more information.