String initialContext = "java:comp/env";
Context env = (Context) new InitialContext().lookup(initialContext);
Object o = env.lookup(name);

Use this code in try block
          Context initContext = new InitialContext();
          Context envContext = (Context) initContext.lookup("java:comp/env");
          DataSource ds = (DataSource) envContext.lookup("jdbc/UsersDB");
          Connection connection = ds.getConnection();
**add this on context. xml**
    <Resource
     name="jdbc/UsersDB"
    auth="Container"
    type="javax.sql.DataSource"
    maxActive="100"
    maxIdle="30"
    maxWait="10000"
    driverClassName="com.microsoft.sqlserver.jdbc.SQLServerDriver"
    url="jdbc:sqlserver://192.168.1.56:1433;DatabaseName=testdb1;"
    username="sa"
    password="sa@sql"
    />
**web.xml**
 <resource-ref>
    <description>DB Connection</description>
    <res-ref-name>jdbc/UsersDB</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
</resource-ref>