MySQL and Java - Memory issue

I have a mysql database which get filled with an object that contains a Map with a timestamp (long) and an object (safeboxForLongString).

The String in the safebox is from 5-15MB big.

My Problem is that after ~30 minutes of requesting the String data from the server my RAM Memory is already at 11-12 GB. I tried already to call System.gc and clear the previous filled TreeMap. But no success.

Isn't the purpose of a backed database that I don't have to use all my RAM?

I just cannot finish the process of collecting the server data because I always fail at that problem. Please help me.

Here the relevant code

   //fill the Database
    Main.getAllAvailableEntries().forEach(entryName -> {

        final long[] id = {0};
        treeMapWrapperRepo.findAll().forEach(entry -> {
            if (entry.getCurrency().equals(entryName)) {
                id[0] = entry.getMultiHashMapWrapper_id();
            }
        });

        if (id[0] == 0) {
            System.out.println("Forming: " + entryName);
            final TreeMapWrapper treeMapWrapper = new TreeMapWrapper(entryName);
            TreeMap<Long, SafeBoxForLongString> timeStampToSafeBoxMap = new TreeMap<>();
            int start = startDateInThePast;
            while (start - length >= 0) {
                long startDate = instant.minus(start, ChronoUnit.DAYS).getEpochSecond();
                long endDate = instant.minus(start - length, ChronoUnit.DAYS).getEpochSecond();

                String getReponseWith45DaysLength = dataController.getDataForDB(entryName, startDate, endDate, String.valueOf(resolution));
                while (getReponseWith45DaysLength.isEmpty()) {
                    getReponseWith45DaysLength = dataController.getDataForDB(entryName, startDate, endDate, String.valueOf(resolution));
                }

                SafeBoxForLongString safeBoxForLongString = new SafeBoxForLongString(getReponseWith45DaysLength);
                safeBoxForLongString.setMultiHashMapWrapper(treeMapWrapper);

                timeStampToSafeBoxMap.put(startDate, safeBoxForLongString);
                treeMapWrapper.setLongSafeTimeAndReponseMap(timeStampToSafeBoxMap);

                start--;
            }
            treeMapWrapperRepo.save(treeMapWrapper);
            treeMapWrapper.getLongSafeTimeAndReponseMap().clear();
            System.gc();
        }
    });

Edit: Like one stacker said, treeMapWrapperRepo.findAll(). was the problem.