Question

I have a Java application that uses lots of java.sql.Connection to a database.

I want to test that, if the database is unavailable, my services return the appropriate error codes (distinguishing between temporary and permanent problems e.g. HTTP 500 and 503).

For testing, my application connects to an embedded, local, in-memory h2 database; the application is not aware of this, only my integration test is.

How can I make writes to the database fail deterministically, e.g. hook into commits and make them throw a custom SQLException? I want a global ‘database is unavailable’ boolean in the test code that affects all connections and makes my application exercise its reconnect logic.

^{(I had started by proxying Connection and putting an if(failFlag) throw new MySimulateFailureException() in commit(); but this didn’t catch PreparedStatement.executeUpdate(); before I embark on proxying the PreparedStatement too – its a lot of methods! – I’d like to be taught a better way…)}

Answer 1

I ended up making my own Java reflection wrapper that intercepts Connection.commit and the PreparedStatement.execute... methods.

My final code in my ‘DBFactory’ class:

@SuppressWarnings("serial")
public class MockFailureException extends SQLException {
    private MockFailureException() {
        super("The database has been deliberately faulted as part of a test-case");
    }
}

private class MockFailureWrapper implements InvocationHandler {

    final Object obj;

    private MockFailureWrapper(Object obj) {
        this.obj = obj;
    }

    @Override public Object invoke(Object proxy, Method m, Object[] args) throws Throwable {
        if(dbFailure && ("commit".equals(m.getName()) || m.getName().startsWith("execute")))
            throw new MockFailureException();
        Object result;
        try {
            result = m.invoke(obj, args);
            if(result instanceof PreparedStatement)
                result = java.lang.reflect.Proxy.newProxyInstance(
                        result.getClass().getClassLoader(),
                        result.getClass().getInterfaces(),
                        new MockFailureWrapper(result));
        } catch (InvocationTargetException e) {
            throw e.getTargetException();
        } catch (Exception e) {
            throw new RuntimeException("unexpected invocation exception: " + e.getMessage());
        }
        return result;
    }

}


public Connection newConnection() throws SQLException {
    Connection connection = DriverManager.getConnection("jdbc:h2:mem:"+uuid+";CREATE=TRUE;DB_CLOSE_ON_EXIT=FALSE");
    return (Connection)java.lang.reflect.Proxy.newProxyInstance(
            connection.getClass().getClassLoader(),
            connection.getClass().getInterfaces(),
            new MockFailureWrapper(connection));
}