I have a main controller servlet in which i instantiate a datasource. The servlet opens and closes the connections. Mainly, the servlet instantiates a command from the application using the "factory pattern". here is some code to explain:
public void init() throws ServletException {
super.init();
try {
datasource =(DataSource) getServletContext().getAttribute("DBCPool");
}
catch (Exception e) {
}
}
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
//some code...
Connection connection = null;
if(cmd.mightNeedLazyLoadingAConnection)
{
connection = null;
}
else
connection = getConnection();//where getConnection is a method: datasource.getconnection();
//now a command (a java class) is instantied, to which the "null" CONNECTION obj is passed as parameter
cmdFactory.getInstance().getCommand(Cmd).execute(tsk,connection);
//some code
//Then wherever there is catch exception i close() the connection
// and it is always closed in finally
finally {
if(connection!=null)
connection.close()
}
}
Now , this has a problem for the first case, ie connection=null, as it does never close the connection in the "finally" part (explained why in Update below).
"connection=null" is for cases where commands might not need to open a db connection because the data it is seeking for are cached in an identity map.
I tried to pass the "Connection" obj as a "null" parameter in the .execute(tsk,connection); and then in the corresponding java class to open a connection if needed
--> it did open the connection inside the command, however when process goes back to servlet : "Connection" is null as thus not closed.
What can i do to make the "Connection" obj's value get updated so that when back in servlet it is not "Null" anymore and i'd be able to close it?
I generally prefer to use a controller servlet that opens/close db connections, so what would be the best way to deal this kind of scenario where you have to do some sort of "lazy loading" a db connection from the pool and at the same time keep the opens/close of db connection assigned to the servlet?
Update (to explain further):
say i have a command : X.java
this command might/might not need a db connection (depends if the data searched for are in the identity map or not)
The system that i would like to have is:
(1)"client request"
(2)---> "Servlet": command.execute(connection)//where connection = null for now
(3) ---> "Command X": Do i need to go to database or record is in identity map?
(3.a) Case where it is needed to go to the database:
(3.a.1)connection = datasource.getconnection
(3.a.2) go get the data
(4)--->back to servlet: close "connection" in "Servlet"
Right now it is working until (3.a.2), but once back in (4) it appears that connection is still "null" and thus the code:
finally {
if(connection!=null)
connection.close()
}
Does not work (doesn't close the connection) and thus the db pool get drained like that.
How could connection - which starts as "null" and changes inside command "X"- get "globaly" updated to the its new value, and not only "updated" inside the scope of command "X"?
SOLUTION(S)
In case you are encountering the same scenario, you can chose of these 2 solutions:
You can Either use LazyConnectionDataSourceProxy, as mentionned by #Ryan Stewart for a "clean abstraction" and more professional solution
Or if you'd like use my solution described below (Basically i implemented a class similar to "LazyConnectionDataSourceProxy" but it is not as clean, it has less abstraction of details than "LazyConnectionDataSourceProxy")
My personal solution, Details:
I created a "Helper" class, which constructor takes the "datasource" as parameter
This helper class has methods to: "Lazy get" connection from pool,"close" connection
This class is instantiated in the servlet, and it gets a connection from the pool Only if needed throughout the application.
This is the code i added/modified in the servlet:
Connection connection = null;
if(cmd.mightNeedLazyLoadingAConnection)
{
helper hp = new helper(datasource);
cmdFactory.getInstance().getCommand(Cmd).execute(tsk,hp);
}
else
{
connection = getConnection();
cmdFactory.getInstance().getCommand(Cmd).execute(tsk,connection);
}
Then say in a command "X" , a db connection is needed i do:
Connection connection = hp.LazyGet();//Now got a connection from the pool
And this way, when proccess flow is back to the servlet level, i can :
Close
rollback
commit
etc..
All on this hp object of the helper class.
What benefits do i get from this:
I limit all database open / close / commit / rollback in one place, ie the Servlet, which is responsible of executing commands.
Having 3 cases: never needs db / always needs db / might need db thus now i decreased calls to the database by 1/3 , which is quite a lot knowing that database call grows exponentially with new features and new users registrations.
It might not be the Cleanest workaround, but between this way and having an additional "unnecessary" 1/3 database calls, it surely is better. Or just use LazyConnectionDataSourceProxy if you want a Tested, abstract and clean method.
Use a LazyConnectionDataSourceProxy. Then just get a "connection" every time, but a real connection is opened only when you actually do something that requires one. Thus you obey the "create/destroy" wisdom which Hiro2k pointed out because the connection's lifecycle is completely managed by your servlet.
In your specific case the only way to do it would be to return the connection. Java doesn't have any pass by reference semantics that could help you, unlike C where you could pass in the reference to the connection and then set it within the method.
I don't recommend your method return the connection, instead remember this simple rule and everything will work as you expect:
The object that creates it, is responsible for destroying it.
If what you want to do is not instantiate a connection for commands that don't require one, then add a method to your command interface that simply returns if needs one.
Command command = cmdFactory.getInstance().getCommand(Cmd);
if(command.requiresConnections){
connection = getConnection();
}
command.execute(tsk,connection);
Related
I am implementing a program that needs to establish connection to mysql database. I'm currently connecting independently for each class (like sign up, sign in, show users etc), using the following lines:
Class.forName("com.mysql.cj.jdbc.Driver");
con = DriverManager.getConnection("jdbc:mysql://localhost:3308/myfirstdb","root");
What I am thinking is to implement a class for the connection and call its default constructor each time. Does it give any advantages/disadvantages or doesn't matter. This is the class:
public class MyConnection {
private Connection con;
public MyConnection() {
//establishing connection
try {
Class.forName("com.mysql.cj.jdbc.Driver");
con = DriverManager.getConnection("jdbc:mysql://localhost:3308/myfirstdb","root","");
} catch (ClassNotFoundException | SQLException e) {
e.printStackTrace();
}
}
//for future use
public Connection getCon() {
return con;
}
}
With very rare exceptions, a single connection to the database for the entire program is the "correct" thing to do.
Connecting costs something. All your queries can use that one connection.
Sure, have the "constructor" do the "connect". And have the "destructor" do "disconnect".
The class should be a "singleton" (or equivalent) to avoid accidentally getting two connections.
Your code has public MyConnection(), which begs the user to create multiple connections. That should be private and called only once, such as this way:
public Connection getCon() {
if (con == NULL) { // or whatever the syntax is
MyConnection(); // Here's the _one_ connection
}
return con;
}
Then users call getCon() to get the one connection and use it to perform queries.
Beware: If you language facilitates multiple threads, do one of these:
Do database operations only from one thread, or
Have (at most) one connection per thread.
PS: I believe this advice applies to any OO language.
As a footnote, establishing two connections in an attempt to get extra performance is likely to be futile. Or, at least, not worth the effort.
Another thing to be aware of... HTTP is, mostly, stateless. So, if you have one web page to "sign up", that will come and go -- one connection with some number of SQLs, then it goes away. Another web page to "sign in" will involve another HTTP request, another connection, etc.
After that, the memory that the user is "signed in" need to be held somewhere:
In URL parameters -- subject to hacking
In a cookie -- reasonable for lightweight apps
Other -- You need security advice if working with sensitive (credit card, health care, etc) info and need to go from web page to web page.
This is a solved problem in Java, don't try to write this yourself. Use a javax.sql.DataSource implementation that is backed by a connection pool (for example Apache DBCP, HikariCP or c3p0).
You can then obtain a connection from the data source for a unit-of-work, and close the connection when you're done. Closing the connection will return the connection to the connection pool for re-use, eliminating much of the overhead of opening a connection.
I'm creating app for library management with Java and MySQL ( JDBC to connect with DB ) , and I have a problem , I checked a lot of topics, books, and websites but I didn't find good answer for me. Is it the good way to deal with connections ? I think that one connection for entire app is good option in this case. My idea is that in every function in every class when I need to use Connection object , these functions will need a connection parameter. In main class I'll call manager object 'Man' for example and to every constructor etc I'll pass Man.getMyConn() as this parameter and call Man.close() when Main frame will be closed . Is it bad idea ? Maybe I should use singleton pattern or connection pool ?
Sorry for my English , I'm still learning.
public class manager {
private Connection myConn;
public manager() throws Exception {
Properties props = new Properties();
props.load(new FileInputStream("app.properties"));
String user = props.getProperty("user");
String password = props.getProperty("password");
String dburl = props.getProperty("dburl");
myConn = DriverManager.getConnection(dburl, user, password);
System.out.println("DB connection successful to: " + dburl);
}
public Connection getMyConn() {
return myConn;
}
//close class etc.
}
Usually not. Further answer depends on type of the application. If you're making web application then you should definitely go with connection pool. If you're making e.g. desktop application (where only one user can access it at the time), then you can open and close connection upon each request.
I have working applications that do it your way. As #Branislav says, it's not adequate if you want to do multiple concurrent queries. There's also a danger that the connection to the database might be lost, and you would need to restart your application to get a new one, unless you write code to catch that and recreate the connection.
Using a singleton would be overcomplicated. Having a getConnection() method (as you have done) is very important as it means you can easily change your code to use a pool later if you find you need to.
We are currently dealing with the function, that has to work partially with the database, and partially with a service, whice operations are time-consuming. So, generally speaking, here is a transactional method, that has a code like this:
Connection conn = null;
try {
conn = getConnection(); // This I get from the connection pool
Employee emp = queryDatabase(id);
// Point A - ??
Response resp = makeLongTimeServiceCall(emp);
// Point B - ??
anotherQueryDatabase(resp);
} catch (Exception e) {
throw e; // And this also rolls back the transaction
} finally {
// If conn is not closed, close it, so there would be no leaks
}
So the big question is - should I close the connection at point A and then get it again from the connection pool at point B, so that other servlets could use that connection while I interact with the service? My guess is that I should, but will this hurt my transaction?
In many circumstances: yes, closing and reopening the connection sounds good. However you need to understand the implication of doing them in two separate transactions (by closing and re-opening the connection you are inherently doing them in separate transaction).
If another user invokes an operation that changes the state of your data at Point B will the end result still be correct?
If you don't have a transaction you can close the connection and ask for a new connection when needed.
Remember that if you are using a connection pool closing a connection will not really close the connection, but only flag it as reusable by other threads.
Database connections shouldn't be left open. Open the connections just when you need to execute a query and close it as early as possible. So my solution is, YES.
if your application doesn't close connection properly may lead to some issues like the
connection pool maxing out.
Applications Not Properly Closing Connections:
When writing an application to use the WebSphere Application Server(WAS) datasource, the best way is fallowing get/use/close pattern.
1.get
-This is when the application makes a request to get a connection from the pool.
The application will first lookup the datasource name and then do a getConnection() call.
2.use
-This is when the application executes a query and waits for a response
3.close
-This is the final stage when the application has received the response from the database and is now done using that connection.
-The application should call close() in a finally block to close out the connection and return it to the free pool.
If your application does not follow this pattern, you may see connections staying open for long periods of time. This is especially seen when the application does not close the connection properly. After the response is received from the database, the application should be calling close in a finally block. If the application does not explicitly close the connection, that connection will stay open forever in WAS until that server is restarted. This is known as a connection leak. To resolve this type of issue, you will have to modify your application to close every connection
for further information: https://www.ibm.com/support/pages/common-reasons-why-connections-stay-open-long-period-time-websphere-application-server
I have a multithreaded application which uses OracleDriver.defaultConnection() to connect to the database.
This line of code is used in multiple code blocks which are accessed by different threads.
I am getting the error : Java Thread Deadlock detected
This is a sample piece of code :
private void function1() {
//something here
Connection conn = OracleDriver.defaultConnection();
// something here
conn.execute();
conn.close();
}
Second piece of code which is in a separate class :
private void function2() {
//something here
Connection conn = OracleDriver.defaultConnection();
// something here
conn.execute();
conn.close();
}
How do I make this connection "thread-safe"?
Also, I read that defaultConnection() returns a static object. So can this be caused due to the connection being closed by one thread while the other thread is using it?
I cannot use any connection method which requires me to input the database username and password.
Thanks in advance!
Java provides many ways for dealing with concurrency(synchronized, ReentrantLock, ReadWriteLock)
Considering the documentation which says:
The oracle.jdbc.OracleDriver class defaultConnection() method is an Oracle extension and always returns the same connection object.
Since it always returns the same connection object you can synchronize on it:
Connection conn = OracleDriver.defaultConnection();
synchronized (conn) {
// something here
conn.execute();
}
However you would need to remove all conn.close() calls due to:
If you do call the close() method, be aware of the following: All connection instances obtained through the defaultConnection() method, which actually all reference the same connection object, will be closed and unavailable for further use, with state and resource cleanup as appropriate. Executing defaultConnection() afterward would result in a new connection object.
Even though the connection object is closed, the implicit connection to the database will not be closed.
Our standard code section for using JDBC is...
Connection conn = getConnection(...);
Statement stmt = conn.conn.createStatement (ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_READ_ONLY);
ResultSet rset = stmt.executeQuery (sqlQuery);
// do stuff with rset
rset.close(); stmt.close(); conn.close();
Question 1: When using Connection Pool, should one close the Connection at the end? If so, isn't the purpose of pooling lost? And if not, how does the DataSource know when a particular instance of Connection is freed up and can be reused? I am a little confused on this one, any pointers appreciated.
Question 2: Is the following method anything close to standard? Looks like an attempt to get a connection from the pool, and if DataSource cannot be established, use the old fashioned DriverManager. We are not even sure which part is getting executed at runtime.
Repeating the question above, should one close the Connection coming out of such a method?
synchronized public Connection getConnection (boolean pooledConnection)
throws SQLException {
if (pooledConnection) {
if (ds == null) {
try {
Context envCtx = (Context)
new InitialContext().lookup("java:comp/env");
ds = (DataSource) envCtx.lookup("jdbc/NamedInTomcat");
return ds.getConnection();
} catch (NamingException e) {
e.printStackTrace();
}}
return (ds == null) ? getConnection (false) : ds.getConnection();
}
return DriverManager.getConnection(
"jdbc:mysql://"+ipaddy+":"+dbPort +"/" + dbName, uName, pWord);
}
Edit: I think we are getting the pooled connection since we do not see a stack trace.
When using Connection Pool, should one close the Connection at the end? If so, isn't the purpose of pooling lost? And if not, how does the DataSource know when a particular instance of Connection is freed up and can be reused? I am a little confused on this one, any pointers appreciated.
Yes, certainly you need to close the pooled connection as well. It's actually a wrapper around the actual connection. It wil under the covers release the actual connection back to the pool. It's further up to the pool to decide whether the actual connection will actually be closed or be reused for a new getConnection() call. So, regardless of whether you're using a connection pool or not, you should always close all the JDBC resources in reversed order in the finally block of the try block where you've acquired them. In Java 7 this can be further simplified by using try-with-resources statement.
Is the following method anything close to standard? Looks like an attempt to get a connection from the pool, and if DataSource cannot be established, use the old fashioned DriverManager. We are not even sure which part is getting executed at runtime. Repeating the question above, should one close the Connection coming out of such a method?
The example is pretty scary. You just need to lookup/initialize the DataSource only once during application's startup in some constructor / initialization of an applicationwide DB config class. Then just call getConnection() on the one and same datasource throughout the rest of application's lifetime. No need for synchronization nor nullchecks.
See also:
Is it safe to use a static java.sql.Connection instance in a multithreaded system?
Am I Using JDBC Connection Pooling?
The pools typically return you a wrapped Connection object, where the close() method is overridden, typically returning the Connection to the pool. Calling close() is OK and probably still required.
A close() method would probably look like this:
public void close() throws SQLException {
pool.returnConnection(this);
}
For your second question, you could add a logger to show whether the bottom block ever runs. I would imagine though you'd only want one way or the other for the configuration of your database connections. We solely use a pool for our database accesses. Either way, closing the connection would be pretty important to prevent leaks.
Actually, the best approach to connection management is to not farm them out to any code anywhere.
Create a SQLExecutor class that is the one and only location which opens and closes connections.
The entire rest of the application then pumps statements into the executor rather than getting connections from the pool and managing (or mismanaging them) all over the place.
You can have as many instances of the executor as you want, but no one should be writing code that opens and closes connections on its own behalf.
Conveniently, this also lets you log all your SQL from a single set of code.