23 Mar 2016
First post on commons-pool
In my own project, we need to connect to dif databases according to the env user chooses(the url can be anyone), so it seems mybatis or other database orm framework, not support these feature(because we need to config url in properties file, can not assign when the program is running), so I designed to maintain my own connection pool, and I finished a simple connection pool within several hours(the implementation will be introduced later), then I find it is a common issue to maintain a object pool if the object’s creation will cost a lot of resources, and for connection, commons-dbcp is a well designed framework, and it is based on commons-pool, commons-pool is a framework for maintaining dif objects, so I designed to learn it from detail.
#Implementation of my own connection pool
Interface explain:
Connections is an interface which contains:
Connection getConnection(String url, String db);
void releaseConnection(String url, String db, Connection connection)// maybe it should not be called release
Implementation explain:
Fields explain:
ConnectionsImpl which implements Connections contains:
private final int POOL_SIZE = 3; // init connection number for
private final String URL_FORMAT = "jdbc:mysql://%s:3306/%s"; // first ph is for url, second for db
// key in map is for "url"_"db", list is the 3 connections for each url_db,
Map<String, List<Connection>> connections = new HashMap<String, List<Connection>>();
###public Connection getConnection(String url, String db)
//generate the complete url
String completeUrl = String.format(URL_FORMAT, url, db);
//init connection pool for the specified "url"_"db", just for the first time,
//we create 3 connections for each.
//just the first thread which call getConnection one this url, db will enter the sync block
//so we add this if condition to avoid sync every time when a thread call getConnection().
if (connections.get(url + "_" + db) == null) {
//lock the whole map, and add three connections on this url_db
synchronized (connections) {
if (connections.get(url + "_" + db) == null) {
try {
ArrayList<Connection> urlConnections = new ArrayList<Connection>();
Class.forName("com.mysql.jdbc.Driver").newInstance();
Connection con = null;
for (int i = 0; i < POOL_SIZE; i++) {
con = DriverManager.getConnection(completeUrl, "username", "password");
urlConnections.add(con);
}
connections.put(url + "_" + db, urlConnections);
} catch (Exception e) {
throw new DBException("db error", e);
}
}
}
}
###return a connection on the specified url_db
//sync on the list of the url_db
List<Connection> connectionsDB = connections.get(url + "_" + db);
synchronized (connectionsDB) {
if (connectionsDB.size() > 0) {
//get the first connection, remove it from the connections list then return the connection
Connection connection = connectionsDB.get(0);
connectionsDB.remove(connection);
return connection;
} else {
try {
// if the connection list of this url_db contains no connection currently, the getConnection thread
// need to wait for other thread which has connection to release
connectionsDB.wait();
// get again
return getConnection(url, db);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
return null;
###public void releaseConnection(String url, String db, Connection connection) #####for the first time I forget to call this method to return the connection back to the connection list of url_db #####then, for each app, we can just get 3 times of each url_db, then frontend will show status pending, because #####the backend thread on this ajax call is waiting for available connection of the url_db.
List<Connection> connectionsDB = connections.get(url + "_" + db);
synchronized (connectionsDB) {
//add the connection back to the url_db list, then notify the thread waiting for the url_db connection
connectionsDB.add(connection);
connectionsDB.notify();
}
###public void destroyConnections()
//never used it should be called when the tomcat is shutdowning, but I do not consider this because, the
//app is small, we can just ignore this 3 connections on each url_db.
Iterator it = connections.entrySet().iterator();
while (it.hasNext()) {
Map.Entry pair = (Map.Entry) it.next();
Iterator itConnection = ((List)pair.getValue()).iterator();
while (itConnection.hasNext()) {
Connection con = (Connection) itConnection.next();
try {
con.close();
} catch (SQLException e) {
throw new DBException("close connection error", e);
}
}
}
Learning travel～
2016.03.23
xule
