Question

Consider an object declared in a method:

public void foo() {
    final Object obj = new Object();

    // A long run job that consumes tons of memory and 
    // triggers garbage collection
}

Will obj be subject to garbage collection before foo() returns?

UPDATE:
Previously I thought obj is not subject to garbage collection until foo() returns.

However, today I find myself wrong.

I have spend several hours in fixing a bug and finally found the problem is caused by obj garbage collected!

Can anyone explain why this happens? And if I want obj to be pinned how to achieve it?

Here is the code that has problem.

public class Program
{
    public static void main(String[] args) throws Exception {
        String connectionString = "jdbc:mysql://<whatever>";

        // I find wrap is gc-ed somewhere
        SqlConnection wrap = new SqlConnection(connectionString); 

        Connection con = wrap.currentConnection();
        Statement stmt = con.createStatement(ResultSet.TYPE_FORWARD_ONLY, 
             ResultSet.CONCUR_READ_ONLY);
        stmt.setFetchSize(Integer.MIN_VALUE);

        ResultSet rs = stmt.executeQuery("select instance_id, doc_id from
               crawler_archive.documents");

        while (rs.next()) {
            int instanceID = rs.getInt(1);
            int docID = rs.getInt(2);

            if (docID % 1000 == 0) {
                System.out.println(docID);
            }
        }

        rs.close();
        //wrap.close();
    }
}

After running the Java program, it will print the following message before it crashes:

161000
161000
********************************
Finalizer CALLED!!
********************************
********************************
Close CALLED!!
********************************
162000
Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: 

And here is the code of class SqlConnection:

class SqlConnection
{
    private final String connectionString;
    private Connection connection;

    public SqlConnection(String connectionString) {
        this.connectionString = connectionString;
    }

    public synchronized Connection currentConnection() throws SQLException {
        if (this.connection == null || this.connection.isClosed()) {
            this.closeConnection();
            this.connection = DriverManager.getConnection(connectionString);
        }
        return this.connection;
    }

    protected void finalize() throws Throwable {
        try {
            System.out.println("********************************");
            System.out.println("Finalizer CALLED!!");
            System.out.println("********************************");
            this.close();
        } finally {
            super.finalize();
        }
    }

    public void close() {
        System.out.println("********************************");
        System.out.println("Close CALLED!!");
        System.out.println("********************************");
        this.closeConnection();
    }

    protected void closeConnection() {
        if (this.connection != null) {
            try {
                connection.close();
            } catch (Throwable e) {
            } finally {
                this.connection = null;
            }
        }
    }
}

Answer 1

As your code is written the object pointed to by “wrap” shouldn’t be eligible for garbage collection until “wrap” pops off the stack at the end of the method.

The fact that it is being collected suggests to me your code as compiled doesn’t match the original source and that the compiler has done some optimisation such as changing this:

SqlConnection wrap = new SqlConnection(connectionString); 
Connection con = wrap.currentConnection();

to this:

Connection con = new SqlConnection(connectionString).currentConnection();

(Or even inlining the whole thing) because “wrap” isn’t used beyond this point. The anonymous object created would be eligible for GC immediately.

The only way to be sure is to decompile the code and see what’s been done to it.