JDBC | Chapter 5: closing and releasing JDBC connection resources

Because it is not like Java Programs that do self - managing memory and garbage collection , So when we use database resources , To manually manage the use and release of these resources . If we use some resources , But did not release , Database resources will be occupied all the time , When there's a lot of data , Will drag our program down . This requires us to write relevant programs , When we finish using a resource , We should take the initiative to release it .

What resources do we need to manually manage in the database ？ There are mainly 3 individual ,Connection、Statement、ResultSet, These three types of resources should be closed after they are used , Otherwise it will be reported ORA-00604 And ORA-01000 Such a mistake .

And it's important to note that , Note that code exceptions may cause these resources not to be released , So the release code of these resources needs to be put into finally{ } In the code block .

Question 1 ：

Connection、Statement and ResulSet？ These three objects are defined inside the method , Then the three objects disappear after the method is executed , Why close them alone ？
answer ：
This connection is a connection to the database server , Although your method is over , But the resource still exists and the database connection has not been released

Question two ：

Why is it JDBC After accessing the database , To press close first ResultSet, Then close it PreparedStatement, Finally close Connection, Direct closure Connection No, that's fine ？

answer ：

1. It seems as if you just put connection It's closed , The system can run normally . That's in queries or other operations , If only close Connection, Don't make ResultSet and Statement If it's closed , Will it affect the system performance . Or other adverse effects of reality .
   If you don't use connection pooling , So there's no problem , once Connection close , The database physical connection is released , All the relevant Java Resources can also be GC Recycled . But if you use connection pooling , So please pay attention ,Connection Shutdown is not a physical shutdown , Just return the connection pool , therefore PreparedStatement and ResultSet All held , And actually occupy the cursor resources of the relevant database , under these circumstances , As long as it runs for a long time , Often report “ The cursor exceeds the maximum allowed in the database ” Error of , Cause the program to fail to correct Often access the database

2. Because you open it in order ,
   When open ：Connection -> PreparedStatement -> ResultSet
   closed ：ResultSet-> PreparedStatement -> Connection
   This is like a stack , Last in, first out

Resource release code

finally {
            // Remember to close the database connection resource after executing the database operation 
            try {
                resultSet.close();
                preparedStatement1.close();
                preparedStatement2.close();
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }