How should I use try-with-resources with JDBC?

问题:

I have a method for getting users from a database with JDBC:

public List<User> getUser(int userId) {
    String sql = \"SELECT id, name FROM users WHERE id = ?\";
    List<User> users = new ArrayList<User>();
    try {
        Connection con = DriverManager.getConnection(myConnectionURL);
        PreparedStatement ps = con.prepareStatement(sql); 
        ps.setInt(1, userId);
        ResultSet rs = ps.executeQuery();
        while(rs.next()) {
            users.add(new User(rs.getInt(\"id\"), rs.getString(\"name\")));
        }
        rs.close();
        ps.close();
        con.close();
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return users;
}

How should I use Java 7 try-with-resources to improve this code?

I have tried with the code below, but it uses many try, and doesn\'t improve the readability much. Should I use try-with-resources in another way?

public List<User> getUser(int userId) {
    String sql = \"SELECT id, name FROM users WHERE id = ?\";
    List<User> users = new ArrayList<>();
    try {
        try (Connection con = DriverManager.getConnection(myConnectionURL);
             PreparedStatement ps = con.prepareStatement(sql);) {
            ps.setInt(1, userId);
            try (ResultSet rs = ps.executeQuery();) {
                while(rs.next()) {
                    users.add(new User(rs.getInt(\"id\"), rs.getString(\"name\")));
                }
            }
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return users;
}

回答1:

I realize this was long ago answered but want to suggest an additional approach that avoids the nested try-with-resources double block.

public List<User> getUser(int userId) {
    try (Connection con = DriverManager.getConnection(myConnectionURL);
         PreparedStatement ps = createPreparedStatement(con, userId); 
         ResultSet rs = ps.executeQuery()) {

         // process the resultset here, all resources will be cleaned up

    } catch (SQLException e) {
        e.printStackTrace();
    }
}

private PreparedStatement createPreparedStatement(Connection con, int userId) throws SQLException {
    String sql = \"SELECT id, username FROM users WHERE id = ?\";
    PreparedStatement ps = con.prepareStatement(sql);
    ps.setInt(1, userId);
    return ps;
}

回答2:

There\'s no need for the outer try in your example, so you can at least go down from 3 to 2, and also you don\'t need closing ; at the end of the resource list. The advantage of using two try blocks is that all of your code is present up front so you don\'t have to refer to a separate method:

public List<User> getUser(int userId) {
    String sql = \"SELECT id, username FROM users WHERE id = ?\";
    List<User> users = new ArrayList<>();
    try (Connection con = DriverManager.getConnection(myConnectionURL);
         PreparedStatement ps = con.prepareStatement(sql)) {
        ps.setInt(1, userId);
        try (ResultSet rs = ps.executeQuery()) {
            while(rs.next()) {
                users.add(new User(rs.getInt(\"id\"), rs.getString(\"name\")));
            }
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return users;
}

回答3:

Here is a concise way using lambdas and JDK 8 Supplier to fit everything in the outer try:

    try (Connection con = DriverManager.getConnection(JDBC_URL, prop);
            PreparedStatement stmt = ((Supplier<PreparedStatement>)() -> {
                try {
                  PreparedStatement s = con.prepareStatement(
                        \"SELECT userid, name, features FROM users WHERE userid = ?\");
                  s.setInt(1, userid);
                  return s;
                } catch (SQLException e) { throw new RuntimeException(e); }
            }).get();
          ResultSet resultSet = stmt.executeQuery()) {
    }

回答4:

What about creating an additional wrapper class?

package com.naveen.research.sql;

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

public abstract class PreparedStatementWrapper implements AutoCloseable {

    protected PreparedStatement stat;

    public PreparedStatementWrapper(Connection con, String query, Object ... params) throws SQLException {
        this.stat = con.prepareStatement(query);
        this.prepareStatement(params);
    }

    protected abstract void prepareStatement(Object ... params) throws SQLException;

    public ResultSet executeQuery() throws SQLException {
        return this.stat.executeQuery();
    }

    public int executeUpdate() throws SQLException {
        return this.stat.executeUpdate();
    }

    @Override
    public void close() {
        try {
            this.stat.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Then in the calling class you can implement prepareStatement method as:

try (Connection con = DriverManager.getConnection(JDBC_URL, prop);
    PreparedStatementWrapper stat = new PreparedStatementWrapper(con, query,
                new Object[] { 123L, \"TEST\" }) {
            @Override
            protected void prepareStatement(Object... params) throws SQLException {
                stat.setLong(1, Long.class.cast(params[0]));
                stat.setString(2, String.valueOf(params[1]));
            }
        };
        ResultSet rs = stat.executeQuery();) {
    while (rs.next())
        System.out.println(String.format(\"%s, %s\", rs.getString(2), rs.getString(1)));
} catch (SQLException e) {
    e.printStackTrace();
}