FMDB与多线程的研究

• 前言

  • SQLITE线程安全, 与FMDB多线程安全是两回事;

  • SQLITE默认的线程模式是串行模式, 是线程安全的

  • FMDatabase多线程不安全, 单个FMDatabaseQueue是多线程安全的;

多线程安全测试

• FMDatabase 多线程

    NSString *dbPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).lastObject stringByAppendingPathComponent:@"tmps.db"];
    
    FMDatabase *db = [FMDatabase databaseWithPath:dbPath];
    
    [db open];
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
    
        for (NSInteger i = 0; i < 10; i++) {
            
            [db executeUpdate:@"INSERT INTO t_PetInfo(petNo, age, name, weight, hoster) VALUES(?, ?, ?, ?, ?);", @(i), @(2), @"myPet0", @(15.0), @"derain"];
        }
        
    });
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        
        for (NSInteger i = 0; i < 10; i++) {
            
            [db executeUpdate:@"INSERT INTO t_PetInfo(petNo, age, name, weight, hoster) VALUES(?, ?, ?, ?, ?);", @(i), @(2), @"myPet1", @(15.0), @"derain"];
        }
 
    });
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        

        for (NSInteger i = 0; i < 10; i++) {
            
            [db executeUpdate:@"INSERT INTO t_PetInfo(petNo, age, name, weight, hoster) VALUES(?, ?, ?, ?, ?);", @(i), @(2), @"myPet2", @(15.0), @"derain"];
        }

    });
    
    // 打印
2017-08-28 19:37:20.547 ABDataBase[22333:3419905] The FMDatabase <FMDatabase: 0x60000008f910> is currently in use.
2017-08-28 19:37:20.547 ABDataBase[22333:3419908] The FMDatabase <FMDatabase: 0x60000008f910> is currently in use.
2017-08-28 19:37:20.547 ABDataBase[22333:3419905] The FMDatabase <FMDatabase: 0x60000008f910> is currently in use.
2017-08-28 19:37:20.547 ABDataBase[22333:3419908] The FMDatabase <FMDatabase: 0x60000008f910> is currently in use.
2017-08-28 19:37:20.548 ABDataBase[22333:3419905] The FMDatabase <FMDatabase: 0x60000008f910> is currently in use.

• 测试结论: 数据插入不全
  • FMDatabase 多线程不安全

image.png

• FMDatabaseQueue 多线程

    dispatch_async(dispatch_get_global_queue(0, 0), ^{
    
        for (NSInteger i = 0; i < 5; i++) {
            
            [self.queue inDatabase:^(FMDatabase *db) {
                
                [db executeUpdate:@"INSERT INTO t_MyTable(name, age, no, class_id) VALUES(?, ?, ?, ?);", [NSString stringWithFormat:@"derain1"], @(20 + arc4random_uniform(10)), @(i), @(arc4random_uniform(10))];
            }];
        }
        
    });
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        
        for (NSInteger i = 0; i < 5; i++) {
            
            [self.queue inDatabase:^(FMDatabase *db) {
                
                [db executeUpdate:@"INSERT INTO t_MyTable(name, age, no, class_id) VALUES(?, ?, ?, ?);", [NSString stringWithFormat:@"derain2"], @(20 + arc4random_uniform(10)), @(i), @(arc4random_uniform(10))];
            }];
        }
 
    });
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        
        for (NSInteger i = 0; i < 5; i++) {
            
            [self.queue inDatabase:^(FMDatabase *db) {
                
                [db executeUpdate:@"INSERT INTO t_MyTable(name, age, no, class_id) VALUES(?, ?, ?, ?);", [NSString stringWithFormat:@"derain3"], @(20 + arc4random_uniform(10)), @(i), @(arc4random_uniform(10))];
            }];
        }
    });
    

• 测试结论: 数据全部插入成功
  • FMDatabaseQueue 多线程安全

image.png

• FMDatabaseQueue 与 多线程 与 事务

    dispatch_async(dispatch_get_global_queue(0, 0), ^{

        [self.queue inTransaction:^(FMDatabase *db, BOOL *rollback) {
             
            for (NSInteger i = 0; i < 5; i++) {
                
                [db executeUpdate:@"INSERT INTO t_MyTable(name, age, no, class_id) VALUES(?, ?, ?, ?);", [NSString stringWithFormat:@"derain0"], @(20 + arc4random_uniform(10)), @(i), @(arc4random_uniform(10))];
            }
                
        }];

    });
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        
        [self.queue inTransaction:^(FMDatabase *db, BOOL *rollback) {
            
            for (NSInteger i = 0; i < 5; i++) {
                
                [db executeUpdate:@"INSERT INTO t_MyTable(name, age, no, class_id) VALUES(?, ?, ?, ?);", [NSString stringWithFormat:@"derain1"], @(20 + arc4random_uniform(10)), @(i), @(arc4random_uniform(10))];
            }
            
        }];
 
    });
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        
        [self.queue inTransaction:^(FMDatabase *db, BOOL *rollback) {
            
            for (NSInteger i = 0; i < 5; i++) {
                
                [db executeUpdate:@"INSERT INTO t_MyTable(name, age, no, class_id) VALUES(?, ?, ?, ?);", [NSString stringWithFormat:@"derain2"], @(20 + arc4random_uniform(10)), @(i), @(arc4random_uniform(10))];
            }
            
        }];
    });

• 为什么FMDatabaseQueue能实现多线程安全
  • 其多线程是在不同子线程把任务追加_queue中
  • 真正操作数据库的任务还是有_queue来分配
  • _queue是一个串行队列, 且是同步执行, 所以所有任务是一个接一个执行, 并不会造成资源抢夺

- (void)inDatabase:(void (^)(FMDatabase *db))block {
    
    dispatch_sync(_queue, ^() {
        
        FMDatabase *db = [self database];
        
        block(db);
        
    FMDBRelease(self);
}