连接关闭不会关闭Tomcat中的连接

Question

In my java web project which runs on Tomcat there are over 4000 sleeping connections after 2 day (I checked them with sp_who command). I do close every statement, resultset and connections after I do a database job. I use the below template for database stuffs.

    try{
        this.openConnection();
        this.createStatement();

        // things ...

        this.cleanResources();
    }catch (SQLException e){
        this.cleanResources();
        e.printStackTrace();
    }

    public void cleanResources() {
    try {
        if (this.rs != null) {
            rs.close();
            this.rs = null;
        }
        if (this.stmt != null) {
            stmt.close();
            this.stmt = null;
        }
        if (this.conn != null) this.closeConnection();
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        if (this.conn != null) this.closeConnection();
    }
}

   public void closeConnection() {
    try {
        if (this.conn != null)
            this.conn.close();
        this.isOpen = false;
    } catch (Exception e) {
        e.printStackTrace();
    }
}

public void createStatement() {
    try {
        if (!this.isOpen) this.openConnection();
        this.stmt = this.conn.createStatement();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

public void openConnection() {
    try {
        this.conn = ds.getConnection(); // ds is a javax.sql.DataSource instance
        this.isOpen = true;
    } catch (Exception e) {
        e.printStackTrace();
    }
}

After every time a sql code worked, the row counts which has sleeping status does increases and the database stuffs gets so much slower. Why does it happens? How can I completely kill a connection after I done with it? Is it really the sleeping connections that makes slower my SQL Server?

Here is my Tomcat configurations(In context.xml):

maxTotal="20" maxActive="20" maxIdle="20"

Answer 1

Your code / pattern for cleaning connections is flawed:

try {
    this.openConnection();
    this.createStatement();

    // things ...

    this.cleanResources();
} catch (SQLException e){
    this.cleanResources();
    e.printStackTrace();
}

This doesn't close connections if "things" throws some exception that is not an SQLException or subclass.

If should be:

try {
    this.openConnection();
    this.createStatement();

    // things ...

    this.cleanResources();
} catch (SQLException e) {
    e.printStackTrace();
} finally {
    this.cleanResources();
}

Then this:

public void cleanResources() {
    try {
        if (this.rs != null) {
            rs.close();
            this.rs = null;
        }
        if (this.stmt != null) {
            stmt.close();
            this.stmt = null;
        if (this.conn != null) this.closeConnection();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

If either rs.close() or stmt.close() throws an exception, then the connection is not closed.

It should be something like this:

public void cleanResources() {
    try {
        if (this.rs != null) {
            rs.close();
            this.rs = null;
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    try {
        if (this.stmt != null) {
            stmt.close();
            this.stmt = null;
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    try {
        if (this.conn != null) this.closeConnection();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Either of these flaws could result in leaking of database connections in some cases.

Another possibility is that somewhere in your code you are not following your (flawed) pattern.

I think you need to read up on:

• Java 7+ "try with resources" support
• Java 1.0+ "try ... finally".

---

It is also worth noting that it is bad practice to:

• catch Exception ... in most cases
• squash exceptions (ie catch and silently continue) ... in most cases
• scatter calls to printStackTrace throughout your code.