Description:
When a Statement has been closed/is closing there is a small time frame where calls to execute() can cause NullPointerException. In StatementImpl:
If thread A closes the Statement and the thread is at a point before this row in realClose():
this.isClosed = true;
Thread B can then enter a execute() method and make it into the core of it, passing the checkClosed() check point.
Thread A then continues to execute and sets this.isClosed to true and then beginning to null the instance variables.
Thread B then continues to execute inside the execute method and tries to use the instance variables (the exact location of the NullPointerException may vary.
A Statement can be closed for a couple for reasons one example is that another thread invokes cancel(). Another might be that the Connection object is closed and close is propagates to all the opened Statement objects.
Here are some example stack traces:
With Mysql connector 5.0.8:
java.lang.NullPointerException
at com.mysql.jdbc.PreparedStatement.fillSendPacket(PreparedStatement.java:1709)
at com.mysql.jdbc.PreparedStatement.fillSendPacket(PreparedStatement.java:1685)
at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:1396)
at se.lesc.database.DAOSql.getByKey(DAOSql.java:80)
Exception in thread "Timer-0" java.lang.NullPointerException
at com.mysql.jdbc.Statement.execute(Statement.java:585)
at se.lesc.database.DAOSql.getBySql(DAOSql.java:45)
java.lang.NullPointerException
at com.mysql.jdbc.Statement.execute(Statement.java:742)
at se.lesc.mysql_close_npe.CloseNPETest$1.run(CloseNPETest.java:34)
at java.lang.Thread.run(Unknown Source)
With Mysql connector 5.1.7:
java.lang.NullPointerException
at com.mysql.jdbc.StatementImpl.execute(StatementImpl.java:794)
at com.mysql.jdbc.StatementImpl.execute(StatementImpl.java:587)
at se.lesc.mysql_close_npe.CloseNPETest$1.run(CloseNPETest.java:34)
at java.lang.Thread.run(Unknown Source)
How to repeat:
Run the following test program
public class CloseNPETest {
@Test
public void testNPE() throws ClassNotFoundException, SQLException, InterruptedException {
//Dummy class to store an exception from a thread.
class ExeceptionContainer {
private Exception e;
}
final ExeceptionContainer exceptionContainer = new ExeceptionContainer();
Class.forName("com.mysql.jdbc.Driver");
Connection connection = DriverManager.getConnection("jdbc:mysql://localhost/test", "root", "");
final Statement statement = connection.createStatement();
Thread myThread = new Thread(new Runnable() {
public void run() {
try {
for (int i = 0; i < 100000; i++) {
try {
//Execute a dummy statement
statement.execute("SELECT 1+2");
} catch (SQLException e) {
//This is okay
}
}
} catch (RuntimeException e) {
//Store the exception
exceptionContainer.e = e;
}
}
});
myThread.start();
//Ensure that the thread has been started
Thread.sleep(1000);
//Perform "evil" close
statement.close();
myThread.join();
if (exceptionContainer.e != null) {
exceptionContainer.e.printStackTrace();
fail("RuntimeException from execute");
}
}
}
The program must be modified to use correct database, user and password. The exact location of the NullPointerException might vary. It does not occur all the times. I executed the test on
* Intel duel core CPU, Windows XP, Mysql server backend 5.0
* Intel single core CPU, Windows XP, Mysql server backend 5.1
The first two exceptions example in the bug report was seen on a production system running Linux with Intel Dual Core CPU (Mysql-server backend 5.0).
Suggested fix:
I guess a "closing"-state could be introduced in realClose. This closing state must probably be synchronized. This state can then be used to determine if other threads should enter the execute methods (via the checkClosed() method). This fix is probably not trivial since care must be taken because the isClosed instance variable is used in sub classes.
Description: When a Statement has been closed/is closing there is a small time frame where calls to execute() can cause NullPointerException. In StatementImpl: If thread A closes the Statement and the thread is at a point before this row in realClose(): this.isClosed = true; Thread B can then enter a execute() method and make it into the core of it, passing the checkClosed() check point. Thread A then continues to execute and sets this.isClosed to true and then beginning to null the instance variables. Thread B then continues to execute inside the execute method and tries to use the instance variables (the exact location of the NullPointerException may vary. A Statement can be closed for a couple for reasons one example is that another thread invokes cancel(). Another might be that the Connection object is closed and close is propagates to all the opened Statement objects. Here are some example stack traces: With Mysql connector 5.0.8: java.lang.NullPointerException at com.mysql.jdbc.PreparedStatement.fillSendPacket(PreparedStatement.java:1709) at com.mysql.jdbc.PreparedStatement.fillSendPacket(PreparedStatement.java:1685) at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:1396) at se.lesc.database.DAOSql.getByKey(DAOSql.java:80) Exception in thread "Timer-0" java.lang.NullPointerException at com.mysql.jdbc.Statement.execute(Statement.java:585) at se.lesc.database.DAOSql.getBySql(DAOSql.java:45) java.lang.NullPointerException at com.mysql.jdbc.Statement.execute(Statement.java:742) at se.lesc.mysql_close_npe.CloseNPETest$1.run(CloseNPETest.java:34) at java.lang.Thread.run(Unknown Source) With Mysql connector 5.1.7: java.lang.NullPointerException at com.mysql.jdbc.StatementImpl.execute(StatementImpl.java:794) at com.mysql.jdbc.StatementImpl.execute(StatementImpl.java:587) at se.lesc.mysql_close_npe.CloseNPETest$1.run(CloseNPETest.java:34) at java.lang.Thread.run(Unknown Source) How to repeat: Run the following test program public class CloseNPETest { @Test public void testNPE() throws ClassNotFoundException, SQLException, InterruptedException { //Dummy class to store an exception from a thread. class ExeceptionContainer { private Exception e; } final ExeceptionContainer exceptionContainer = new ExeceptionContainer(); Class.forName("com.mysql.jdbc.Driver"); Connection connection = DriverManager.getConnection("jdbc:mysql://localhost/test", "root", ""); final Statement statement = connection.createStatement(); Thread myThread = new Thread(new Runnable() { public void run() { try { for (int i = 0; i < 100000; i++) { try { //Execute a dummy statement statement.execute("SELECT 1+2"); } catch (SQLException e) { //This is okay } } } catch (RuntimeException e) { //Store the exception exceptionContainer.e = e; } } }); myThread.start(); //Ensure that the thread has been started Thread.sleep(1000); //Perform "evil" close statement.close(); myThread.join(); if (exceptionContainer.e != null) { exceptionContainer.e.printStackTrace(); fail("RuntimeException from execute"); } } } The program must be modified to use correct database, user and password. The exact location of the NullPointerException might vary. It does not occur all the times. I executed the test on * Intel duel core CPU, Windows XP, Mysql server backend 5.0 * Intel single core CPU, Windows XP, Mysql server backend 5.1 The first two exceptions example in the bug report was seen on a production system running Linux with Intel Dual Core CPU (Mysql-server backend 5.0). Suggested fix: I guess a "closing"-state could be introduced in realClose. This closing state must probably be synchronized. This state can then be used to determine if other threads should enter the execute methods (via the checkClosed() method). This fix is probably not trivial since care must be taken because the isClosed instance variable is used in sub classes.