Java 用于从连接池获取连接的静态类

我想了解连接池的概念

 <Resource name="jdbc/appname"
              auth="Container"
              type="javax.sql.DataSource"
              maxActive="100" 
              maxIdle="30"
              maxWait="10000"
              minIdle="10"
              username="postgres" 
              password="123"
              driverClassName="org.postgresql.Driver"
              url="jdbc:postgresql://localhost:5432/Lab4"/>

这就是用法

public class UserQueries{ 
 public User selectUserByLoginAndPassword(final String login, final String password) {
        try(Connection connection=DataBaseConnection.getConnection())
        try (PreparedStatement st = connection.prepareStatement(SELECT_QUERY)) {
            st.setString(1, login);
            st.setString(2, password);
            ResultSet result = st.executeQuery();
            while (result.next()) {
                final User user = User.newBuilder()
                        .setId(result.getInt("id"))
                        .setAge(result.getInt("age"))
                        .setName(result.getString("name"))
                        .setPassword(result.getString("password"))
                        .setLogin(login)
                        .build();
                return user;

            }
        } catch (SQLException ex) {
            throw new RuntimeException(ex);
        }
        throw new NullPointerException("Nu such user in db");
    }

   }

问题是，如果我用以下方式修改DataBaseConnection

public class DataBaseConnection {

        private  DataSource dataSource;

        public DataBaseConnection()
            try {
                InitialContext initContext = new InitialContext();
                dataSource = (DataSource) initContext.lookup("java:comp/env/jdbc/appname");
            } catch (NamingException ex) {
                throw new RuntimeException(ex);                
        }

        public  Connection getConnection() {
            try {
                return dataSource.getConnection();
            } catch (SQLException ex) {
                throw new RuntimeException(ex);
            }
        }

我将在使用db连接的每个类中创建新的DataBaseConnection对象，这是否意味着每个类（例如UserQueries）将创建单独的连接池来使用

范例

public class UserQueries{ 
  private DataBaseConnection dbCon = new DataBaseConnection();
 public User selectUserByLoginAndPassword(final String login, final String password) {
        try(Connection connection = dbCon.getConnection())
        try (PreparedStatement st = connection.prepareStatement(SELECT_QUERY)) {
            st.setString(1, login);
            st.setString(2, password);
            ResultSet result = st.executeQuery();
            while (result.next()) {
                final User user = User.newBuilder()
                        .setId(result.getInt("id"))
                        .setAge(result.getInt("age"))
                        .setName(result.getString("name"))
                        .setPassword(result.getString("password"))
                        .setLogin(login)
                        .build();
                return user;

            }
        } catch (SQLException ex) {
            throw new RuntimeException(ex);
        }
        throw new NullPointerException("Nu such user in db");
    }

   }

---

每个JNDI查找都会返回

数据源的一个新实例。每个
DataSource
实例都维护自己的连接池

请参见下面的
Datasource.getConnection（）
的tomcat实现：

public Connection getConnection() throws SQLException {
    if (pool == null)
        return createPool().getConnection();
    return pool.getConnection();
}

这将创建
连接池
：

private synchronized ConnectionPool pCreatePool() throws SQLException {
    if (pool != null) {
        return pool;
    } else {
        pool = new ConnectionPool(poolProperties);
        return pool;
    }
}

总之


每次JNDI查找都会返回新的
DataSource
实例
将为
数据源的每个实例创建新的连接池

这是否意味着使用静态数据源可以保证我的应用程序只使用一个连接池？更好的说法是：使用单个
数据源
实例是可取的方法。“…我的应用程序将只使用一个连接池？”--是。
private synchronized ConnectionPool pCreatePool() throws SQLException {
    if (pool != null) {
        return pool;
    } else {
        pool = new ConnectionPool(poolProperties);
        return pool;
    }
}