Key steps in executing JDBC queries include loading the driver and establishing a connection, creating a Statement and executing a query, processing a result set, and closing resources. First, the database driver must be loaded and a connection must be established. For example, when using MySQL, the driver is loaded through Class.forName() and the connection is obtained with DriverManager; then create a Statement through conn.createStatement() and call executeQuery() to execute a SELECT query; then iterate over the ResultSet to extract the field values, note that the field name must match the database column name; finally, be sure to close the ResultSet, Statement and Connection. It is recommended to use try-with-resources to automatically manage resources to prevent leakage.
Executing a JDBC query is not complicated, but you have to write the details correctly. First, you have to make sure that the database driver has been loaded and the connection has been established before you can actually execute the query statement.
Load the driver and establish a connection
Before executing the query, the database's JDBC driver must be loaded and the connection must be obtained through DriverManager . For example, if you use MySQL, the code is roughly like this:
Class.forName("com.mysql.cj.jdbc.Driver");
Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb", "user", "password"); This step is the basics. If the driver class name is written incorrectly or the URL is connected incorrectly, the program will not be able to run. The URL format is generally determined by the database type, such as PostgreSQL is jdbc:postgresql://... , Oracle is jdbc:oracle:thin:@... , etc.
Create a Statement and execute a query
After you have a connection, you can create Statement object to execute SQL. The commonly used method is to call createStatement() method and then use executeQuery() to execute SELECT query:
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT id, name FROM users"); Note that you should use executeQuery() instead of executeUpdate() , which is used to perform modification operations such as INSERT, UPDATE, or DELETE. If you use the wrong method, an exception will be thrown.
Processing result sets
The query returns a ResultSet , which is like a cursor, pointing to the data of the current row. You need to loop through each row and extract the field values:
while (rs.next()) {
int id = rs.getInt("id");
String name = rs.getString("name");
System.out.println("ID: " id ", Name: " name);
} Here you need to pay attention to whether the case of the field name matches the actual column name of the database. Some databases (such as PostgreSQL) convert column names to lowercase by default, so using rs.getString("Name") may not find the field.
Also, remember to close the resources after each use up. Although many connection pools will help you with it now, it is a good habit to manually close ResultSet , Statement , and Connection without using the connection pool:
rs.close(); stmt.close(); conn.close();
Tips: Using try-with-resources is safer
If you are using Java 7 and above, you can use try-with-resources to automatically close the resources to avoid forgetting to close an object:
try (Connection conn = DriverManager.getConnection("jdbc:mysql://...", "user", "password");
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT...")) {
while (rs.next()) {
// Process data}
}This writing is not only more concise, but also effectively prevents resource leakage.
Basically that's it. Writing JDBC queries seems to have many steps, but each step is clear. The key is to handle connections and resource releases well.
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