I found this code in a tutorial but it isn't working when I debug it try the connection and then it isn't throw exception only go to the finally block without do any line of code under the DriverManager.getConnection().
Why? Anyone has an idea?
Connection con = null;
Statement st = null;
ResultSet rs = null;
String url = "jdbc:mysql://host/databasename";
String user = "user";
String password = "pass";
try {
Class.forName("com.mysql.jdbc.Driver");
} catch (ClassNotFoundException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
try {
con = (Connection) DriverManager.getConnection(url, user, password);
st = (Statement) con.createStatement();
rs = st.executeQuery("SELECT * FROM Message");
if (rs.next()) {
System.out.println(rs.getString(1));
}
} catch (SQLException ex) {
Logger lgr = Logger.getLogger(Version.class.getName());
lgr.log(Level.SEVERE, ex.getMessage(), ex);
} catch (java.sql.SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}
finally {
try {
if (rs != null) {
rs.close();
}
if (st != null) {
st.close();
}
if (con != null) {
con.close();
}
} catch (SQLException ex) {
Logger lgr = Logger.getLogger(Version.class.getName());
lgr.log(Level.WARNING, ex.getMessage(), ex);
} catch (java.sql.SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
Logcat write this:
Logcat Message
1) DriverManager.getConnection() returns an object of type Connection, so there is no need to cast it. Same thing for st = (Statement) con.createStatement();
2) Unless you're running MySQL on a remote machine, you need to make sure you have MySQL installed on your local machine. If you decide to run MySQL on your local machine, you can connect to it with String url = "jdbc:mysql://localhost/{existing_db_name}"; given that everything else remains the same.
3) It seems like you're catching the same exception twice:
try{
con = (Connection) DriverManager.getConnection(url, user, password);
...
}catch (SQLException ex) {
Logger lgr = Logger.getLogger(Version.class.getName());
lgr.log(Level.SEVERE, ex.getMessage(), ex);
} catch (java.sql.SQLException e) {
e.printStackTrace();
}
As a result, your second catch block, the one that prints the exception message to System.err, never runs. Instead, you print the exception message to a Logger. This might be why you think no exception is being thrown, when in reality, an exception is being thrown.
4) Make sure you download the JDBC driver for MySQL. Copy and paste it into the directory of your project.
I've deleted some catch block:
Connection con = null;
Statement st = null;
ResultSet rs = null;
String url = "jdbc:mysql://host:3306/databasename";
String user = "user";
String password = "pass";
try {
Class.forName("com.mysql.jdbc.Driver");
} catch (ClassNotFoundException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
try {
con = (Connection) DriverManager.getConnection(url, user, password);
st = (Statement) con.createStatement();
rs = st.executeQuery("SELECT * FROM Message");
if (rs.next()) {
System.out.println(rs.getString(1));
}
} catch (SQLException ex) {
ex.printStackTrace();
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}
finally {
try {
if (rs != null) {
rs.close();
}
if (st != null) {
st.close();
}
if (con != null) {
con.close();
}
} catch (SQLException ex) {
ex.printStackTrace();
}
}
Can you please specify what String url are you using? At this point, with these modifications I obviously get a SQLException with No suitable driver found for jdbc:mysql://host/databasename
Related
I am trying to see the vulnerability of my code with fortify. The report said that I have an issue which said "the function sometimes fails to release a database resource allocated by". Here is the code and in which line the issue pointed. I've tried to close the connection in the finally block but it not solve the issue. How to fix this?
private AnotherService anotherService;
private void create() {
Connection conn = null;
try {
conn = getCon(); // With fortify, there's an issue which said "the function sometimes fails to release a database resource allocated by", and it refers to this line
conn.setAutoCommit(false);
anotherService.myFunction(conn);
// the conn.commit() is inside anotherService, because I have to make one connection
// rest of code
} catch (Exception e) {
e.printStackTrace;
if (null != conn) {
conn.rollback();
}
} finally {
if (null != conn) {
conn.close();
}
}
}
private static Connection getCon() {
Connection connection = null;
try {
Class.forName("org.postgresql.Driver");
connection = DriverManager.getConnection(
"jdbc:postgresql://localhost:5432/dbname",
"username",
"password");
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
return connection;
}
Addition:
If I use try-with-resource (like this try (Connection conn = getCon()), to automatically close things, how I could call conn.rollback() in the catch block if any exception occured? Since the conn variable declared inside the try-with-resources.
Well, I solve my problem, the close method should call inside try-catch in the finally block, as mentioned in this link.
In case the link broken, here is the code that I use to solve my problem:
Statement stmt = null;
ResultSet rs = null;
Connection conn = getConnection();
try {
stmt = conn.createStatement();
rs = stmt.executeQuery(sqlQuery);
processResults(rs);
} catch (SQLException e) {
// Forward to handler
} finally {
try {
if (rs != null) {rs.close();}
} catch (SQLException e) {
// Forward to handler
} finally {
try {
if (stmt != null) {stmt.close();}
} catch (SQLException e) {
// Forward to handler
} finally {
try {
if (conn != null) {conn.close();}
} catch (SQLException e) {
// Forward to handler
}
}
}
}
Useful piece of code for Hive JDBC:
Connection con = null;
Statement stmt = null
try {
Class.forName("org.apache.hive.jdbc.HiveDriver");
con = DriverManager.getConnection(connectionUri, userName, password);
stmt = con.createStatement();
stmt.executeUpdate(query);
} catch (ClassNotFoundException cex) {
cex.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
} finally {
if (stmt != null) {
try {
stmt.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
if (con != null) {
try {
con.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
}
I want to remove try - catch in finally block.
So I tried The try-with-resources Statement.
try (Class.forName("org.apache.hive.jdbc.HiveDriver");
Connection con = DriverManager.getConnection(connectionUri, userName, password);
Statement stmt = con.createStatement();){
stmt.executeUpdate(query);
} catch (ClassNotFoundException cex) {
cex.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
I think this is not the right way.
Class.forName("org.apache.hive.jdbc.HiveDriver") should not be in try. Should I make a separate try-catch for this?
try {
Class.forName("org.apache.hive.jdbc.HiveDriver");
} catch (ClassNotFoundException cex) {
cex.printStackTrace();
}
try (Connection con = DriverManager.getConnection(connectionUri, userName, password);
Statement stmt = con.createStatement();){
stmt.executeUpdate(query);
} catch (SQLException e) {
e.printStackTrace();
}
Is this right way or am I missing any thing?
The idea behind try-with-ressource is to close an AutoCloseable class.
So every usage of a class which should be closed after using it (a Ressource) can be used with try-with-ressource (like Connection for example). You don't have to take care of closing it manually (in an finally block for example).
So yes, your idea is right:
try/catch for Class.forName("org.apache.hive.jdbc.HiveDriver"); - because this is not AutoCloseable
try-with-ressource for Connection con = DriverManager.getConnection(connectionUri, userName, password);
Statement stmt = con.createStatement();- because Connection and Statement implement AutoCloseable
Reference:
https://docs.oracle.com/javase/7/docs/api/java/lang/AutoCloseable.html
When you're using Java 6 or better and the Apache Hive JDBC driver is JDBC 4 compliant or better* then you do not need the Class.forName("org.apache.hive.jdbc.HiveDriver") stuff at all.
Therefore you can just remove the entire try/catch block from your second solution and you're good to go with just:
try (Connection con = DriverManager.getConnection(connectionUri, userName, password);
Statement stmt = con.createStatement()) {
stmt.executeUpdate(query);
} catch (SQLException e) {
e.printStackTrace();
}
* Which is the case for version 1.2.0 or newer of the Hive JDBC driver
Statement Leakage in JDBC:
pstmt =
StatementLeakage : An open JDBC Statement is not closed on all paths. This can cause a transaction or Statement resources to remain active indefinitely, slowing or preventing access to the database by other requests.: for (object created at line = TunnelDBHandler:139, type = java.sql.PreparedStatement), object used at prepareStatement() # TunnelDBHandler:139
dbManager
.getConnection()
.prepareStatement(
"update TUNNEL_STORE set IS_TUNNEL_OPEN=? where TUNNEL_ID=?");
Add a finally block and call close() on all of your ResultSet(s), Statement(s) and Connection(s). As a very rough example,
Connection conn = null;
Statement stmt = null;
ResultSet rs = null;
try {
// ...
// get a statement for stmt
// get a resultset from the stmt
// ...
while (rs.next()) {
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
if (rs != null) {
try {
rs.close();
} catch (SQLException e) {
}
}
if (stmt != null) {
try {
stmt.close();
} catch (SQLException e) {
}
}
if (conn != null) {
try {
conn.close();
} catch (SQLException e) {
}
}
}
With this (simplified) code example Eclipse (Kepler SR2) gives a warning for the innermost if-statement (if (con != null)), dead code.
public class DbManager {
public String getSingleString(String query) throws SQLException {
DbManager dbmgr = new DbManager();
Connection con = null;
try {
con = dbmgr.getConnection("user", "pwd", URL);
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
if (con != null) {
PreparedStatement pstmt = null;
ResultSet rset = null;
pstmt = con.prepareStatement(query.toString());
rset = pstmt.executeQuery();
if (rset != null && rset.next()) {
return (rset.getString(1));
}
}
}
return null;
}
}
Typically the database connection defined on the line after the try will create a connection and then the offending if-statement will be true. Is the warning about dead code really correct?
If dbmgr.getConnection("user", "pwd", URL); returns an exception, then con will never get assigned a non-null reference.
You initialized con with null. So when an exception will be thrown and your code will reach the catch, con will be null. That is why that check (con != null) does not make sense.
If the connection is successfully created, then that catch statement will never be called so it is dead code, try rearranging it to:
try {
con = dbmgr.getConnection("user", "pwd", URL);
//if (con != null) { <-- not required because of the try and catch
PreparedStatement pstmt = null;
ResultSet rset = null;
pstmt = con.prepareStatement(query.toString());
rset = pstmt.executeQuery();
if (rset != null && rset.next()) {
return (rset.getString(1));
}
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
I realised the problem after running the code a couple of times and bumping into some problems: one } was missing after the catch. It should be:
try {
con = dbmgr.getConnection("cap_x1", "test");
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
if (con != null) {
PreparedStatement pstmt = null;
ResultSet rset = null;
etc. Thank you for your feedback.
After reading through a lot of similar questions I have not been able to get a solution that works for me.
I have this methods:
In a crawler4j Controller I do this:
ArrayList<String> urls = Urls.getURLs(100);
for (String s : urls) {
System.out.println("Adding URL: " + s);
controller.addSeed(s);
}
this is getURLs():
public static ArrayList<String> getURLs(int number) {
ArrayList<String> list = new ArrayList<String>();
String getStatement = "select * from " + Configurations.getStringProperty("mysql.urls.db_name", "urls") + " where retrieved=0 limit "
+ Configurations.getStringProperty("mysql.urls.limit", "100") + ";";
ResultSet rs;
rs = Databaseclient.executeStatement(getStatement);
try {
while (rs.next()) {
list.add(rs.getString("url"));
// Databaseclient.executeStatement("update urls set retrieved = true where id = "
// + rs.getInt("id") + ";");
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
try {
rs.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return list;
}
This is my executeStatement():
public static ResultSet executeStatement(String s) {
Connection connection = null;
Statement statement = null;
ResultSet resultSet = null;
try {
// fetch a connection
connection = DataSource.getInstance().getConnection();
if (connection != null) {
statement = connection.createStatement();
resultSet = statement.executeQuery(s);
}
} catch (SQLException e) {
System.out.println("A SQLException occured executing the Statement");
e.printStackTrace();
} catch (IOException e) {
System.out.println("A IOException occured executing the Statement");
e.printStackTrace();
} catch (PropertyVetoException e) {
System.out.println("A PropertyVetoException occured executing the Statement");
e.printStackTrace();
} finally {
if (resultSet != null) {
try {
resultSet.close();
} catch (SQLException e) {
System.out.println("A SQLException occured executing the Statement");
e.printStackTrace();
}
}
if (statement != null) {
try {
statement.close();
} catch (SQLException e) {
System.out.println("A SQLException occured executing the Statement");
e.printStackTrace();
}
}
if (connection != null) {
try {
connection.close();
} catch (SQLException e) {
System.out.println("A SQLException occured executing the Statement");
e.printStackTrace();
}
}
}
return resultSet;
}
I get the error
java.sql.SQLException: Operation not allowed after ResultSet closed
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1094)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:997)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:983)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:928)
at com.mysql.jdbc.ResultSetImpl.checkClosed(ResultSetImpl.java:799)
at com.mysql.jdbc.ResultSetImpl.next(ResultSetImpl.java:6982)
at database.Urls.getURLs(Urls.java:27)
at crawler.Controller.main(Controller.java:53)
Which is the line
while (rs.next()) {
in my getURLs() method.
What am I doing wrong? There is no code between getting the statement and the while loop that could close the statement.
Your code is a bit off, but if I understand you then don't close your ResultSet in the finally block of your executeStatement method.
public static ResultSet executeStatement(Connection connection,
Statement statement, String s) {
ResultSet resultSet = null;
try {
if (statement != null) {
resultSet = statement.executeQuery(s);
}
} catch (SQLException e) {
System.out.println("A SQLException occured executing the Statement");
e.printStackTrace();
} catch (IOException e) {
System.out.println("A IOException occured executing the Statement");
e.printStackTrace();
} catch (PropertyVetoException e) {
System.out.println("A PropertyVetoException occured executing the Statement");
e.printStackTrace();
}
return resultSet;
}
Then you need to pass in a Connection and Statement, and you'll get a ResultSet back. Also, the caller should then close all three when it's done with the ResultSet.