FMDB 中的多线程处理

对于数据操作,最重要的一点就是数据安全的问题,在多线程中,线程安全是数据安全的首要前提,下面谈谈FMDB 是如何对多线程进行处理的。

FMDB 单例中处理多线程

我们都知道FMDB 一个简单的使用就是调用它的单例模式

FMDatabase *db = [FMDatabase databaseWithPath:@"/tmp/tmp.db"];

先创建个Person 表

NSString *sql = @"create table person (identifier integer primary key autoincrement, name text, age integer);";
BOOL success = [db executeStatements:sql];
if (!success) {
    NSLog(@"error = %@", [db lastErrorMessage]);
}

插入数据的方法

- (BOOL)insertIntoTablePerson:(FMDatabase *)db arguments:(NSDictionary *)arguments {
    BOOL success = [db executeUpdate:@"INSERT INTO person (identifier, name, age) VALUES (:identifier, :name, :age)"
             withParameterDictionary:arguments];

    if (!success) {
        NSLog(@"error = %@", [db lastErrorMessage]);
    }
    return success;
}

多个线程同时使用db会发生什么呢?

dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_group_async(group, queue, ^{
    NSInteger identifier = 0;
    NSString *name = @"Demon";
    NSInteger age = 20;
    NSDictionary *arguments = @{@"identifier": @(identifier),
                                @"name": name,
                                @"age": @(age)};
    if ([self insertIntoTablePerson:db arguments:arguments]) {
        NSLog(@"Demon 插入成功 - %@", [NSThread currentThread]);
    }
});

dispatch_group_async(group, queue, ^{
    NSInteger identifier = 1;
    NSString *name = @"Jemmy";
    NSInteger age = 25;
    NSDictionary *arguments = @{@"identifier": @(identifier),
                                @"name": name,
                                @"age": @(age)};
    if ([self insertIntoTablePerson:db arguments:arguments]) {
        NSLog(@"Jemmy 插入成功 - %@", [NSThread currentThread]);
    }
});

dispatch_group_async(group, queue, ^{
    NSInteger identifier = 2;
    NSString *name = @"Michael";
    NSInteger age = 42;
    NSDictionary *arguments = @{@"identifier": @(identifier),
                                @"name": name,
                                @"age": @(age)};
    if ([self insertIntoTablePerson:db arguments:arguments]) {
        NSLog(@"Michael 插入成功 - %@", [NSThread currentThread]);
    }
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"完成 - %@", [NSThread currentThread]);
    // 查询
    FMResultSet *s = [db executeQuery:@"SELECT * FROM person"];
    while ([s next]) {
        int identifier = [s intForColumnIndex:0];
        NSString *name = [s stringForColumnIndex:1];
        int age = [s intForColumnIndex:2];
        NSLog(@"identifier=%d, name=%@, age=%d", identifier, name, age);
    }
    [db close];
});

结果

2016-05-13 09:36:05.629 UCFMDBText[24362:2319873] The FMDatabase <FMDatabase: 0x7f8232c21f10> is currently in use.
2016-05-13 09:36:05.629 UCFMDBText[24362:2319749] The FMDatabase <FMDatabase: 0x7f8232c21f10> is currently in use.
2016-05-13 09:36:05.630 UCFMDBText[24362:2319873] error = not an error
2016-05-13 09:36:05.630 UCFMDBText[24362:2319749] error = not an error
2016-05-13 09:36:05.630 UCFMDBText[24362:2319758] Jemmy 插入成功 - <NSThread: 0x7f8232d07940>{number = 2, name = (null)}
2016-05-13 09:36:05.633 UCFMDBText[24362:2319700] 完成 - <NSThread: 0x7f8232c081e0>{number = 1, name = main}
2016-05-13 09:36:05.633 UCFMDBText[24362:2319700] identifier=1, name=Jemmy, age=25

可以看到,三条插入语句分别开启了三个线程231987323197492319758,只有Michael 线程2319758 插入成功了,另外两条都提示了

1. The FMDatabase <FMDatabase: 0x7f8232c21f10> is currently in use.
2. error = not an error

查看一下Person 表中的数据

FMResultSet *s = [db executeQuery:@"SELECT * FROM person"];
while ([s next]) {
    int identifier = [s intForColumnIndex:0];
    NSString *name = [s stringForColumnIndex:1];
    int age = [s intForColumnIndex:2];
    NSLog(@"identifier=%d, name=%@, age=%d", identifier, name, age);
}

结果,确实只有Michael 一条数据

2016-05-12 22:14:25.445 UCFMDBText[21523:1631615] identifier=2, name=Michael, age=42

所以,多线程时在FMDB 单例中操作同一张表是存在风险的,可能会造成数据丢失。

FMDatabaseQueue

FMDatabaseQueue 就是FMDB 为线程安全提供的解决方案。

接着上面的例子,使用FMDatabaseQueue 代替单例再插入三条数据

dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_group_async(group, queue, ^{
    FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:@"/tmp/tmp.db"];
    [queue inDatabase:^(FMDatabase *db) {
        if ([db executeUpdate:@"INSERT INTO person VALUES (?, ?, ?)", @0, @"Demon", @20]) {
            NSLog(@"Demon 插入成功 - %@", [NSThread currentThread]);
        }
    }];
});

dispatch_group_async(group, queue, ^{
    FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:@"/tmp/tmp.db"];
    [queue inDatabase:^(FMDatabase *db) {
        if ([db executeUpdate:@"INSERT INTO person VALUES (?, ?, ?)", @1, @"Jemmy", @25]) {
            NSLog(@"Jemmy 插入成功 - %@", [NSThread currentThread]);
        }
    }];
});

dispatch_group_async(group, queue, ^{
    FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:@"/tmp/tmp.db"];
    [queue inDatabase:^(FMDatabase *db) {
        if ([db executeUpdate:@"INSERT INTO person VALUES (?, ?, ?)", @2, @"Michael", @42]) {
            NSLog(@"Michael 插入成功 - %@", [NSThread currentThread]);
        }
    }];
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"完成 - %@", [NSThread currentThread]);
    // 查询
    FMResultSet *s = [db executeQuery:@"SELECT * FROM person"];
    while ([s next]) {
        int identifier = [s intForColumnIndex:0];
        NSString *name = [s stringForColumnIndex:1];
        int age = [s intForColumnIndex:2];
        NSLog(@"identifier=%d, name=%@, age=%d", identifier, name, age);
    }
    [db close];
});

结果

2016-05-13 09:19:42.149 UCFMDBText[24345:2307639] Michael 插入成功 - <NSThread: 0x7fb7dbd242d0>{number = 2, name = (null)}
2016-05-13 09:19:42.216 UCFMDBText[24345:2307634] Jemmy 插入成功 - <NSThread: 0x7fb7dd820600>{number = 3, name = (null)}
2016-05-13 09:19:42.301 UCFMDBText[24345:2307624] Demon 插入成功 - <NSThread: 0x7fb7dbc0b630>{number = 4, name = (null)}
2016-05-13 09:19:42.301 UCFMDBText[24345:2307577] 完成 - <NSThread: 0x7fb7dd803300>{number = 1, name = main}
2016-05-13 09:19:42.302 UCFMDBText[24345:2307577] identifier=0, name=Demon, age=20
2016-05-13 09:19:42.302 UCFMDBText[24345:2307577] identifier=1, name=Jemmy, age=25
2016-05-13 09:19:42.303 UCFMDBText[24345:2307577] identifier=2, name=Michael, age=42

通过上面的运行结果可以看出三条insert 语句全部执行成功,并且分别在三个线程230763923076342307624中,相互之间并没有影响,可见FMDatabaseQueue 可以有效的保证其中执行sql 的线程安全。

    原文作者:UncleChen
    原文地址: https://www.jianshu.com/p/cb60951cdc31
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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