put：put的话是会全部覆盖之前的数据，所以我们如果只是想更新Redis中一个对象的一个字段时，最好保证新对象中其它字段都是之前的值，这样就不会有数据丢失了。

putIfAbsent：这个方法会判断key是否有值，如果没有就直接插入，返回true；如果有值的话就不会插入，返回false。

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.springframework.data.redis.core;

import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.Map.Entry;
import org.springframework.lang.Nullable;

public interface HashOperations<H, HK, HV> {
    Long delete(H key, Object... hashKeys);

    Boolean hasKey(H key, Object hashKey);

    @Nullable
    HV get(H key, Object hashKey);

    List<HV> multiGet(H key, Collection<HK> hashKeys);

    Long increment(H key, HK hashKey, long delta);

    Double increment(H key, HK hashKey, double delta);

    @Nullable
    HK randomKey(H key);

    @Nullable
    Entry<HK, HV> randomEntry(H key);

    @Nullable
    List<HK> randomKeys(H key, long count);

    @Nullable
    Map<HK, HV> randomEntries(H key, long count);

    Set<HK> keys(H key);

    @Nullable
    Long lengthOfValue(H key, HK hashKey);

    Long size(H key);

    void putAll(H key, Map<? extends HK, ? extends HV> m);

    void put(H key, HK hashKey, HV value);

    Boolean putIfAbsent(H key, HK hashKey, HV value);

    List<HV> values(H key);

    Map<HK, HV> entries(H key);

    Cursor<Entry<HK, HV>> scan(H key, ScanOptions options);

    RedisOperations<H, ?> getOperations();
}