Redis源代码分析（二十八）--- object创建和释放redisObject物

          今天的学习更有效率。该Rio分析过，学习之间的另一种方式RedisObject文件，只想说RedisObject有些生成和转换。都是很类似的。列出里面长长的API列表:

/* ------------  API --------------------- */
robj *createObject(int type, void *ptr) /* 最初的创建robj对象方法，后面的创建方法与此相似 */	
robj *createStringObject(char *ptr, size_t len)
robj *createStringObjectFromLongLong(long long value)
robj *createStringObjectFromLongDouble(long double value)
robj *dupStringObject(robj *o)
robj *createListObject(void)
robj *createZiplistObject(void)
robj *createSetObject(void)
robj *createIntsetObject(void)
robj *createHashObject(void)
robj *createZsetObject(void)
robj *createZsetZiplistObject(void)
void freeStringObject(robj *o) /* free Obj中的特定对象，这里free的是r->ptr */
void freeListObject(robj *o)
void freeSetObject(robj *o)
void freeZsetObject(robj *o)
void freeHashObject(robj *o) /* 释放hashObject有2种形式，1个是o-ptr的字典对象，还有1个回事压缩表o->ptr */
void incrRefCount(robj *o) /* robj对象增减引用计数,递增robj中的refcount的值 */
void decrRefCount(robj *o) /* 递减robj中的引用计数。引用到0后。释放对象 */
void decrRefCountVoid(void *o)
robj *resetRefCount(robj *obj)
int checkType(redisClient *c, robj *o, int type) /* 检查robj的类型是否为给定的Type类型 */
int isObjectRepresentableAsLongLong(robj *o, long long *llval)
robj *tryObjectEncoding(robj *o) /* 编码一个robj中的额字符对象，主要是为了省空间 */
robj *getDecodedObject(robj *o) /* 获取解码后的robj */
int compareStringObjectsWithFlags(robj *a, robj *b, int flags)
int compareStringObjects(robj *a, robj *b)
int collateStringObjects(robj *a, robj *b)
int equalStringObjects(robj *a, robj *b)
size_t stringObjectLen(robj *o)
int getDoubleFromObject(robj *o, double *target) /* 从robj中获取double数值 */
int getDoubleFromObjectOrReply(redisClient *c, robj *o, double *target, const char *msg)
int getLongDoubleFromObject(robj *o, long double *target)
int getLongDoubleFromObjectOrReply(redisClient *c, robj *o, long double *target, const char *msg)
int getLongLongFromObject(robj *o, long long *target)
int getLongLongFromObjectOrReply(redisClient *c, robj *o, long long *target, const char *msg)
int getLongFromObjectOrReply(redisClient *c, robj *o, long *target, const char *msg)
char *strEncoding(int encoding)
unsigned long estimateObjectIdleTime(robj *o)
robj *objectCommandLookup(redisClient *c, robj *key) /* obj的查找命令， */
robj *objectCommandLookupOrReply(redisClient *c, robj *key, robj *reply)
void objectCommand(redisClient *c)

从前往后看。第一个创建obj:

/* 最初的创建robj对象方法 */	
robj *createObject(int type, void *ptr) {
    robj *o = zmalloc(sizeof(*o));
    o->type = type;
    o->encoding = REDIS_ENCODING_RAW;
    o->ptr = ptr;
    o->refcount = 1;

    /* Set the LRU to the current lruclock (minutes resolution). */
    o->lru = server.lruclock;
    return o;
}

有创建就必定会有释放的free方法:

/* free Obj中的特定对象 */
void freeStringObject(robj *o) {
    if (o->encoding == REDIS_ENCODING_RAW) {
        sdsfree(o->ptr);
    }
}

free方法有非常多衍生的方法。看你要释放哪种类型的空间了，能够，set,dict,ziplist等等。有以下的一些类型:

 switch(o->type) {
        case REDIS_STRING: freeStringObject(o); break;
        case REDIS_LIST: freeListObject(o); break;
        case REDIS_SET: freeSetObject(o); break;
        case REDIS_ZSET: freeZsetObject(o); break;
        case REDIS_HASH: freeHashObject(o); break;
        default: redisPanic("Unknown object type"); break;
        }

重点介绍里面的关于引用计数的相关方法，通过robj->refcount的数值进行控制的:

/* robj对象增减引用计数,递增robj中的refcount的值 */
void incrRefCount(robj *o) {
	//递增robj中的refcount的值
    o->refcount++;
}

添加引用计数就一行代码，可是递减的话。我们猜也能够猜到，引用计数变为0的时候，说明无人使用了，就能够释放空间了；

/* 递减robj中的引用计数，引用到0后。释放对象 */
void decrRefCount(robj *o) {
	//假设之前的引用计数已经<=0了，说明出现异常情况了
    if (o->refcount <= 0) redisPanic("decrRefCount against refcount <= 0");
    if (o->refcount == 1) {
    	//假设之前的引用计数为1，再递减一次。恰好内有被不论什么对象引用了，所以就能够释放对象了
        switch(o->type) {
        case REDIS_STRING: freeStringObject(o); break;
        case REDIS_LIST: freeListObject(o); break;
        case REDIS_SET: freeSetObject(o); break;
        case REDIS_ZSET: freeZsetObject(o); break;
        case REDIS_HASH: freeHashObject(o); break;
        default: redisPanic("Unknown object type"); break;
        }
        zfree(o);
    } else {
    	//其它对于>1的引用计数的情况，仅仅须要按常规的递减引用计数就可以
        o->refcount--;
    }
}

标准的引用计数法控制内存的管理。(提醒一下，在JVM中的对象的生命周期管理用的是根搜索法。不是引用计数)。另一个在robj中的编码方法的实现也是定义在这个文件里:

/* Try to encode a string object in order to save space */
/* 编码一个robj中的额字符对象。主要是为了省空间 */
robj *tryObjectEncoding(robj *o) {
    long value;
    sds s = o->ptr;
    size_t len;

    if (o->encoding != REDIS_ENCODING_RAW)
    	//假设robj已经被编码了，则直接返回
        return o; /* Already encoded */

    /* It's not safe to encode shared objects: shared objects can be shared
     * everywhere in the "object space" of Redis. Encoded objects can only
     * appear as "values" (and not, for instance, as keys) */
     /* 假设robj的引用计数超过1个人引用的时候。是不安全的去编码obj,由于对象是共享的 */
     if (o->refcount > 1) return o;

    /* Currently we try to encode only strings */
    redisAssertWithInfo(NULL,o,o->type == REDIS_STRING);

    /* Check if we can represent this string as a long integer */
    len = sdslen(s);
    if (len > 21 || !string2l(s,len,&value)) {
        /* We can't encode the object...
         *
         * Do the last try, and at least optimize the SDS string inside
         * the string object to require little space, in case there
         * is more than 10% of free space at the end of the SDS string.
         *
         * We do that for larger strings, using the arbitrary value
         * of 32 bytes. This code was backported from the unstable branch
         * where this is performed when the object is too large to be
         * encoded as EMBSTR. */
        if (len > 32 &&
            o->encoding == REDIS_ENCODING_RAW &&
            sdsavail(s) > len/10)
        {
        	//调用sdsRemoveFreeSpace把0->ptr中的字符串中的空格给移除掉
            o->ptr = sdsRemoveFreeSpace(o->ptr);
        }
        /* Return the original object. */
        return o;
    }
    	.....

就是移除字符串中的空格所占的空间。

想相应也存在一个getDecodeObject(),:

/* Get a decoded version of an encoded object (returned as a new object).
 * If the object is already raw-encoded just increment the ref count. */
/* 获取解码后的robj */
robj *getDecodedObject(robj *o) {
    robj *dec;

    if (o->encoding == REDIS_ENCODING_RAW) {
    	//假设没有编码方式。则直接添加引用计数。并返回
        incrRefCount(o);
        return o;
    }
    if (o->type == REDIS_STRING && o->encoding == REDIS_ENCODING_INT) {
        char buf[32];
		//假设是string。Type并且是encoding_int方式的，先做一步转换
        ll2string(buf,32,(long)o->ptr);
        dec = createStringObject(buf,strlen(buf));
        return dec;
    } else {
        redisPanic("Unknown encoding type");
    }
}

以上是对RedisObject简单分析。