Develop Reference

Should I use ThreadLocal to store connections?

I code my Test project and it is prohibited to use Spring and Hibernate there.
I wanted to manage my transactions from Service layer.
For this I have created a class that gets a Connection from the pool and puts it in the ThreadLocal.
This is an example of the fields and the method.
private static ThreadLocal<Connection> threadLocalConnection;
private ComboPooledDataSource comboPooledDataSource ;
public boolean createConnectionIfAbsent() {
boolean isConnectionCreated = false;
try {
Connection currentConnection = threadLocalConnection.get();
if(currentConnection == null) {
Connection conn = this.comboPooledDataSource.getConnection();
conn.setAutoCommit(false);
threadLocalConnection.set(conn);
isConnectionCreated = true;
}
} catch (SQLException e) {
e.printStackTrace();
}
return isConnectionCreated;
}
The class has also close, rollback methods.
Here is the example of how I manage Connections in a Service Layer.
public BigDecimal getTotalOrdersCount() {
boolean connectionCreated = DBManager.getInstance().createConnectionIfAbsent();
BigDecimal ordersCount = BigDecimal.ZERO;
try {
ordersCount = orderDao.getRowNumber();
} catch (SQLException throwables) {
throwables.printStackTrace();
} finally {
if (connectionCreated) DBManager.getInstance().closeConnection();
}
return ordersCount;
}
Dao just uses this to get the connection.
Connection connection = DBManager.getInstance().getConnection();
I found no other way to manage connections in a Servlet project from a Service layer, could you please tell if it is ok? If not - what drawbacks does it have and what should I use instead.
UPD:
Please pay attention to this Service method. Let's assume that Each method in DAO gets the Connection from a pool and closes it.
I do know that I need connection.setAutoCommit(false); to start a transaction, but what to do it in this kind of a situation?
When a single methods calls 2 DAO.
Just give up on a transaction handling?
void setStatusDeclinedAndRefund() {
// sets Order status to DECLINED
// refund money to user's balance
}

No.
Don't second guess the connection pool. Use it in the standard way: get a connection, use it, close it.
There is no need to use the same connection for every database interaction in a given thread. Also, you'll have serious liveliness problems if you allocate each thread a connection, because typically there are way more request processing threads than there are connections in the pool.

Java data transaction performance

I know how to open data transaction with JDBC. But i think I can/must do something to increase data transaction performance. For example:
public class F_Koneksi {
private static final String JDBC_DRIVER;
private static final String DB_URL;
private static final String USER;
private static final String PASS;
static {
JDBC_DRIVER = "org.postgresql.Driver";
DB_URL = "jdbc:postgresql://localhost:5432/MyDatabase";
USER = "Username";
PASS = "Password";
}
private final Connection con;
private ResultSet rs;
private Statement stmt;
public F_Koneksi() {
Connection connect;
try {
Properties props = new Properties();
props.setProperty("user", USER);
props.setProperty("password",PASS);
props.setProperty("sslfactory", "org.postgresql.ssl.NonValidatingFactory");
props.setProperty("ssl", "true");
forName(JDBC_DRIVER);
connect = getConnection(DB_URL, props);
} catch (SQLException|ClassNotFoundException se) {
connect = null;
}
con = connect;
}
public boolean Update(String Query) {
try {
Query = Query.replaceAll("`", "\"");
System.out.println(Query);
Statement stmt = con.createStatement();
stmt.executeUpdate(Query);
return true;
} catch (SQLException ex) {
ex.printStackTrace();
return false;
}
}
And when i must close my connection or turning auto commit off?
What can I do to improve my app data transaction performance? How is the proper way to make data transaction? Or any tips to do it better?

When i must close my connection?
If you are running in a Java EE environment (i.e. on an app server) then you can get and close connections as you wish, since most Java EE environments will pool JDBC connections for you unless you explicitly disable connection pooling.
If you are running in a Java SE environment, this depends on how you are getting the connection. For this example, it looks like you are doing a bunch of static imports (which is bad practice by the way) and you are but from waht I can tell you are using DriverManager to get your connection. If this is true and you are using DriverManager, then getting connections is very expensive! Especially once you start using a remote database. You will want to try to cache your connections. Alternatively, you could use a javax.sql.ConnectionPoolDataSource and use getPooledConnection() which will have much higher performance for get/close scenarios and take care of the connection caching for you.
When should I turn auto-commit on/off?
Auto commit on or off isn't a huge deal. I always like to leave auto-commit on, since it is less error prone by leaving the commit responsibility up to the JDBC driver.
What will help out your performance a lot is if you batch your Statements.
For example:
try(Statement statement = conn.createStatement()){
statement.addBatch("update people set firstname='Alice' where id=1");
statement.addBatch("update people set firstname='Bob' where id=2");
statement.addBatch("update people set firstname='Chuck' where id=3");
statement.executeBatch();
}

Singleton in JSP, how to properly tidy up on close?

I am just getting started with jsp and my question is this - when I have a singleton class, how do I tidy up after it?
In particular:
public class DBConnection {
private static Connection connection = null;
private static Statement statement = null;
public static ResultSet executeQuery(String query){
if (connection == null) { /*initConnection*/ }
if (statement == null) { /*initStatement*/ }
// do some stuff
}
}
Now, I use this class in several pages to get results from jdbc. However, I need to eventually call statement.close(); and connection.close(); - when should I call those?
I am using singleton, because it felt wrong to call for connection to a database over and over whenever I needed to make a query.

The Connection must be closed always, and after you have executed all your database statements for the desired operations. Two examples:
Case 1: You must show a list of products to user filtered by criteria from database. Solution: get a connection, retrieve a list of products using the filter criteria, close the connection.
Case 2: The client selects some of these products and updates the minimum stock to get an alert and restock them. Solution: get a connection, update all the products, close the connection.
Based on these cases, we can learn lot of things:
You can execute more than a single statement while having/maintaining a single connection open.
The connection should live only in the block where it is used. It should not live before or after that.
Both cases can happen at the same time since they are in a multi threaded environment. So, a single database connection must not be available to be used by two threads at the same time, in order to avoid result problems. For example, user A searches the products that are in category Foo and user B searches the products that are in category Bar, you don't want to show the products in category Bar to user A.
From last sentence, each database operation ((or group of similar operations like Case 2) should be handled in an atomic operation. To assure this, the connection must not be stored in a singleton object, instead it must be live only in the method being used.
In consequence:
Do not declare the Connection nor the Statement nor the ResultSet nor other JDBC resource as static. It will simply fail. Instead, declare only the Connection as field of your DBConnection class. Let each method decide to handle each Statement (or PreparedStatement) and ResultSet and specific JDBC resources.
Since you must close the connection after its usage, then add two more methods: void open() and void close(). These methods will handle the database connection retrieval and closing that connection.
Additional, since the DBConnection looks like a wrapper class for Connection class and database connection operations, I would recommend to have at least three more methods: void setAutoCommit(boolean autoCommit), void commit() and void rollback(). These methods will be plain wrappers for Connection#setAutoCommit Connection#close and Connection#rollback respectively.
Then you can use the class in this way:
public List<Product> getProducts(String categoryName) {
String sql = "SELECT id, name FROM Product WHERE categoryName = ?";
List<Product> productList = new ArrayList<Product>();
DBConnection dbConnection = new DBConnection();
try {
dbConnection.open();
ResultSet resultSet = dbConnection.executeSelect(sql, categoryName); //execute select and apply parameters
//fill productList...
} catch (Exception e) {
//always handle your exceptions
...
} finally {
//don't forget to also close other resources here like ResultSet...
//always close the connection
dbConnection.close();
}
}
Note that in this example the PreparedStatement is not in the getProducts method, it will be a local variable of the executeSelect method.
Additional notes:
When working in an application server, you should not open connections naively e.g. using Class.forName("..."), instead use a database connection pool. You can roll on some database connection pooling libraries like C3P0 as explained here: How to establish a connection pool in JDBC?. Or configure one in your application server, as I explain here: Is it a good idea to put jdbc connection code in servlet class?
If this is for learning purposes, then roll on your own classes to handle the communication with your database. In real world applications, this is not recommended (doesn't mean you should not do it). Instead, use a database connectivity framework like ORMs e.g. JPA (Java official ORM framework) or Hibernate; there are no ORM frameworks that handles database communication like Spring JDBC and MyBatis. The choice is yours.
More info:
Should a database connection stay open all the time or only be opened when needed?
How do servlets work? Instantiation, sessions, shared variables and multithreading. Not directly related to your question, but it will help you understand why to not maintain state in resources that are used in multithreaded environments.

Define connection resource in mywebapp/META-INF/context.xml file
<Resource name="jdbc/mydb" auth="Container" type="javax.sql.DataSource"
maxActive="10" maxIdle="2" maxWait="20000"
driverClassName="com.mysql.jdbc.Driver"
username="myuser" password="mypwd"
url="jdbc:mysql://localhost:3306/mydb?useUnicode=true&characterEncoding=utf8"
validationQuery="SELECT 1" />
Create DB.java helper class to minimize code in other parts of app
import java.sql.*;
import javax.sql.DataSource;
import javax.naming.Context;
import javax.naming.InitialContext;
public class DB {
public static Connection createConnection() throws SQLException {
try {
Context ctx = new InitialContext();
DataSource ds = (DataSource)ctx.lookup("java:comp/env/jdbc/mydb");
return ds.getConnection();
} catch (SQLException ex) {
throw ex;
} catch (Exception ex) {
SQLException sqex = new SQLException(ex.getMessage());
sqex.initCause(ex);
throw sqex;
}
}
public static void close(ResultSet rs, Statement stmt, Connection conn) {
if (rs != null) try { rs.close(); } catch (Exception e) { }
if (stmt != null) try { stmt.close(); } catch (Exception e) { }
if (conn != null) try { conn.close(); } catch (Exception e) { }
}
public static void close(ResultSet rs, boolean closeStmtAndConn) {
if (rs==null) return;
try {
Statement stmt = rs.getStatement();
close(rs, stmt, stmt!=null ? stmt.getConnection() : null);
} catch (Exception ex) { }
}
}
And somewhere in your app DAO code use DB helper.
public List<MyBean> getBeans() throws SQLException {
List<MyBean> list = new ArrayList<MyBean>();
ResultSet rs=null;
try {
Connection con = DB.createConnection();
String sql = "Select * from beantable where typeid=?";
PreparedStatement stmt = con.prepareStatement(sql, Statement.NO_GENERATED_KEYS);
stmt.setInt(1, 101);
rs = stmt.executeQuery();
while(rs.next()
list.add( createBean(rs) );
} finally {
DB.close(rs, true); // or DB.close(rs, stmt, conn);
}
return list;
}
private MyBean createBean(ResultSet rs) throws SQLException {
MyBean bean = new MyBean();
bean.setId( rs.getLong("id") );
bean.setName( rs.getString("name" );
bean.setTypeId( rs.getInt("typeid") );
return bean;
}

I would add two methods to the class:
public static void open() throws SomeException;
public static void close() throws SomeException;
then your calling code looks something like this{
try {
DBConnection.open();
... code to use the connection one or more times ...
} finally {
DBConnection.close();
}
Wrap all your database calls inside that and it will take care of closing whether there is an exception thrown or not.
Of course, this isn't much different than having a regular class, which I might recommend:
try {
DBConnection conn = new DBConnection();
conn.open();
... all the code to use the database (but you pass 'conn' around) ...
} finally {
conn.close();
}
And you might want to look at the java.lang.AutoCloseable and java.io.Closeable to see if that helps you.
2
If you are keeping it open across page loads, there isn't any place to put the try ... finally stuff so you can open it and close it when the servlet closes or the server closes or something like that.
If you are going to leave it open, you need to make sure and add code to verify it doesn't close when you aren't looking. A short network glitch, for example, could close it down. In that case, you need to reopen it when it gets closed. Otherwise, all database access from that point will fail.
You might want to look into the concept of a DataBase Pool. Apache has one -- DBCP. Tomcat has its own that's quite good. Other containers, like JBOSS, WebSphere, WebLogic all have them. There's a couple that can be used with the Spring Framework. What it does is manage one or more database connections. Your code asks it for one and it returns an open one, unless none is available and then it opens one and returns it. You call close when your code gets through with it but it doesn't really close the connection, it just returns it to the pool.
You can usually configure the pool to check for shut down connections and reopen if needed.

Custom java.sql.Driver Implementation Connection Handling

Currently, I load the below custom driver (TestDriver.java), get a connection, create a Statement, execute a query, gets the results and close the connection. I open and close a connection for each query. Is this common practice or is there an standard way to share the open connections?
public static void main(String[] args) {
Class.forName("com.sql.TestDriver");
java.sql.Connection conn = DriverManager.getConnection("jdbc:test://8888/connectme", props);
Statement stmt = conn.createStatement;
ResultSet rs = stmt.executeQuery("select * from table");
//loop through rs and pull out needed data
conn.close();
}
public class TestDriver implements java.sql.Driver{
private final TestSchema schema;
private Properties props = null;
static {
try {
DriverManager.registerDriver(new TestDriver());
} catch (SQLException e) {
e.printStackTrace();
}
protected TestDriver() throws SQLException {
schema = TestSchemaFactory.getInstance().getDbSchemaFromFile(SCHEMA_FILE);
//loads in and parses a file containing tables, columns used for business logic
}
public Connection connect(String url, Properties info)
throws SQLException {
TestSqlConnection conn=null;
//connect logic here
return conn; //will return an instance of TestSqlConnection
}
#Override
public boolean jdbcCompliant() {
return false;
}
}

Yes, it's more common to use a database connection pool. This will allow connections to be reused without the overhead or closing/re-opening. Here's a link to DBCP which is one implementation of a database connection pool: http://commons.apache.org/dbcp/

Ideally you should write a separate factory class (can be static)
say ConnectionFactory which returns a connection object.
Also I see that you are not using try/catch/finally block while creating
connection.I strongly suggest to close the connection in finally
clause otherwise you program may suffer from connection leak if any
exception is raised and causes abrupt behavior.
Ideally you should close the connection after your operation is complete in finally
clause.In web based application if you are using connections pool
then closing connection will return the connection back to pool and
will be available for use.

Store database connection as separate Class - Java

Is it possible to store a database connection as a separate class, then call the database objects from a main code? ie;
public class main{
public static void main{
try{
Class.forName("com.jdbc.driver");
Database to = new Database(1,"SERVER1","DATABASE");
Database from = new Database(2,"SERVER2","DATABASE");
String QueryStr = String.format("SELECT * FROM TABLE WHERE Id = %i", to.id)
to.results = sql.executeQuery(QueryStr);
while (to.results.next()) {
String QueryStr = String.format("INSERT INTO Table (A,B) VALUES (%s,%s)",to.results.getString(1),to.results.getString(2));
from.sql.executeQuery("QueryStr");
}
to.connection.close()
from.connection.close()
} catch (Exception ex) {
ex.printStackTrace();
{ finally {
if (to.connection != null)
try {
to.connection.close();
} catch (SQLException x) {
}
if (from.connection != null)
try {
from.connection.close();
} catch (SQLException x) {
}
}
}
public static class Database {
public int id;
public String server;
public String database;
public Connection connection;
public ResultSet results;
public Statement sql;
public Database(int _id, String _server, String _database) {
id = _id;
server = _server;
database = _database;
String connectStr = String.format("jdbc:driver://SERVER=%s;port=6322;DATABASE=%s",server,database);
connection = DriverManager.getConnection(connectStr);
sql = connection.createStatement;
}
}
}
I keep getting a "Connection object is closed" error when I call to.results = sql.executeQuery("SELECT * FROM TABLE"); like the connection closes as soon as the Database is done initializing.
The reason I ask is I have multiple databases that are all about the same that I am dumping into a master database. I thought it would be nice to setup a loop to go through each from database and insert into each to database using the same class. Is this not possible? Database will also contain more methods than shown as well. I am pretty new to java, so hopefully this makes sense...
Also, my code is probably riddled with syntax errors as is, so try not to focus on that.

Connection object is closed doesn't mean that the connection is closed, but that the object relative to the connection is closed (it could be a Statement or a ResultSet).
It's difficult to see from your example, since it has been trimmed/re-arranged, but it looks like you may be trying to use a ResultSet after having re-used its corresponding Statement. See the documentation:
By default, only one ResultSet object per Statement object can be open
at the same time. Therefore, if the reading of one ResultSet object is
interleaved with the reading of another, each must have been generated
by different Statement objects. All execution methods in the Statement
interface implicitly close a statment's current ResultSet object if an
open one exists.

In your example, it may be because autoCommit is set to true by default. You can override this on the java.sql.Connection class. Better yet is to use a transaction framework if you're updating multiple tables.