有没有更简洁的方法来使用try-with-resource和PreparedStatement？

Question

这是Main.java:

package foo.sandbox.db;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class Main {
    public static void main(String[] args) {
        final String SQL = "select * from NVPAIR where name=?";
        try (
                Connection connection = DatabaseManager.getConnection();
                PreparedStatement stmt = connection.prepareStatement(SQL);
                DatabaseManager.PreparedStatementSetter<PreparedStatement> ignored = new DatabaseManager.PreparedStatementSetter<PreparedStatement>(stmt) {
                    @Override
                    public void init(PreparedStatement ps) throws SQLException {
                        ps.setString(1, "foo");
                    }
                };
                ResultSet rs = stmt.executeQuery()
        ) {
            while (rs.next()) {
                System.out.println(rs.getString("name") + "=" + rs.getString("value"));
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

这是 DatabaseManager.java

package foo.sandbox.db;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.Statement;

/**
 * Initialize script
 * -----
 * CREATE TABLE NVPAIR;
 * ALTER TABLE PUBLIC.NVPAIR ADD value VARCHAR2 NULL;
 * ALTER TABLE PUBLIC.NVPAIR ADD id int NOT NULL AUTO_INCREMENT;
 * CREATE UNIQUE INDEX NVPAIR_id_uindex ON PUBLIC.NVPAIR (id);
 * ALTER TABLE PUBLIC.NVPAIR ADD name VARCHAR2 NOT NULL;
 * ALTER TABLE PUBLIC.NVPAIR ADD CONSTRAINT NVPAIR_name_pk PRIMARY KEY (name);
 *
 * INSERT INTO NVPAIR(name, value) VALUES('foo', 'foo-value');
 * INSERT INTO NVPAIR(name, value) VALUES('bar', 'bar-value');
 */
public class DatabaseManager {
    /**
     * Class to allow PreparedStatement to initialize parmaters inside try-with-resource
     * @param <T> extends Statement
     */
    public static abstract class PreparedStatementSetter<T extends Statement> implements AutoCloseable {
        public PreparedStatementSetter(PreparedStatement pstmt) throws SQLException {
            init(pstmt);
        }

        @Override
        public void close() throws Exception {
        }

        public abstract void init(PreparedStatement pstmt) throws SQLException;
    }

    /* Use local file for database */
    private static final String JDBC_CONNECTION = "jdbc:h2:file:./db/sandbox_h2.db;MODE=PostgreSQL";

    static {
        try {
            Class.forName("org.h2.Driver");  // Init H2 DB driver
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * @return Database connection
     * @throws SQLException
     */
    public static Connection getConnection() throws SQLException {
        return DriverManager.getConnection(JDBC_CONNECTION, "su", "");
    }
}

我使用H2数据库是为了简单,因为它是一个易于创建和测试的基于文件的数据库.

因此,所有工作和资源都按预期清理,但我只是觉得可能有更简洁的方法PreparedStatement从try-with-resources块中设置参数(我不想使用嵌套的try/catch块作为那些看起来'尴尬').也许在JDBC中已经存在一个帮助类就是这样,但是我找不到一个.

最好使用lambda函数来初始化PreparedStatement但仍然需要分配一个AutoCloseable对象,以便它可以在try-with-resources中.

Answer 1

首先,你的PreparedStatementSetter课很尴尬:

• 它是一个类型化的类但不使用该类型.
• 构造函数显式调用了一个可覆盖的方法,这是一种不好的做法.

请考虑以下接口(灵感来自同名的Spring接口).

public interface PreparedStatementSetter {
    void setValues(PreparedStatement ps) throws SQLException;
}

这个接口定义了一个PreparedStatementSetter应该做什么的契约:设置a的值PreparedStatement,仅此而已.

然后,最好PreparedStatement在单个方法中创建和初始化内部.在DatabaseManager课堂上考虑这个添加:

public static PreparedStatement prepareStatement(Connection connection, String sql, PreparedStatementSetter setter) throws SQLException {
    PreparedStatement ps = connection.prepareStatement(sql);
    setter.setValues(ps);
    return ps;
}

使用此静态方法,您可以编写:

try (
    Connection connection = DatabaseManager.getConnection();
    PreparedStatement stmt = DatabaseManager.prepareStatement(connection, SQL, ps -> ps.setString(1, "foo"));
    ResultSet rs = stmt.executeQuery()
) {
    // rest of code
}

注意PreparedStatementSetter这里是用lambda表达式写的.这是使用接口而不是抽象类的优势之一:在这种情况下它实际上是一个功能接口(因为有一个抽象方法),因此可以写成lambda.