我在iPhone应用程序中有一个SQLite数据库，我正在编写。我在后台线程中运行以下代码时出错。在后台线程中，我调用这个方法：

- (BOOL) songIsInDatabase:(NSString *)songTitle
{
NSString *docsDir;
NSArray *dirPaths;

dirPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
docsDir = [dirPaths objectAtIndex:0];

//Build the path to the database file
NSString *databasePath = [[NSString alloc] initWithString:[docsDir stringByAppendingPathComponent:@"Database.db"]];

const char *dbpath = [databasePath UTF8String];

sqlite3_stmt *statement;

if(sqlite3_open(dbpath, &DB) == SQLITE_OK){

    NSString *insertSQL = [NSString stringWithFormat:@"select * from Bpm_Table where song_title = '%@'", songTitle];

    const char *insert_stmt = [insertSQL UTF8String];
    if(sqlite3_prepare_v2(DB, insert_stmt, -1, &statement, NULL) == SQLITE_OK)
    {
        while (sqlite3_step(statement) == SQLITE_ROW)
        {
            return YES;
            break;
        }
    }else{
        NSLog(@"the error is %s", sqlite3_errmsg(DB));
    }
    sqlite3_finalize(statement); 
}

[databasePath release];
return NO;
}

然后我调用这个方法：

- (void) addSongToDatabase: (NSString *) songTitle andBPM: (int)bpm andGenre: (NSString *) genre
{
NSString *docsDir;
NSArray *dirPaths;

dirPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
docsDir = [dirPaths objectAtIndex:0];

//Build the path to the database file
NSString *databasePath = [[NSString alloc] initWithString:[docsDir stringByAppendingPathComponent:@"Database.db"]];

const char *dbpath = [databasePath UTF8String];

sqlite3_stmt *statement;

if(sqlite3_open(dbpath, &DB) == SQLITE_OK){

    NSString *insertSQL = [NSString stringWithFormat:@"insert or replace into Bpm_Table (song_title, bpm, genre) values (\"%@\", \"%d\", \"%@\")", songTitle, bpm, genre];

    const char *insert_stmt = [insertSQL UTF8String];
    if(sqlite3_prepare(DB, insert_stmt, -1, &statement, NULL) == SQLITE_OK){
        if(sqlite3_step(statement) == SQLITE_DONE)
        {

        } else {
            NSLog(@"error: %s", sqlite3_errmsg(DB));
        }
    }sqlite3_finalize(statement);
}

[databasePath release];
}

如果我一个接一个地运行这两个方法，我会得到一个错误，说数据库被锁定。我在谷歌周围闲逛后添加了sqlite3_finalize语句，希望能解决这个问题。如果我注解掉这两个方法中的任何一个，我都不会得到这个错误。
有谁知道出了什么问题吗？