Java Out of Memory Error during Encryption

Question

I am using AES to encrypt files. The problem first came when i tried to encrypt a large file. So i did some reading online and figured that i need to use a buffer and only e

Answer 1

Look at these four variables: plainText, bufferedFile, resultByteList, cipherText. All of them contain your entire file in a slightly different format, which means that each of them is 1.2GB big. Two of them are Lists which means they are likely to be even bigger, because you didn't set the initial size of ArrayLists and they resize automatically when needed. So we are talking about over 5GB of memory needed.

Actually, you add chunks into ByteArrayOutputStream baos, which means that it must store it internally, before you call toByteArray() on it. So it's 5 copies of your data, meaning 6GB+. The ByteArrayOutputStream is internally using an array so it grows similarly to ArrayLists so it will use more memory than needed (see the stacktrace - it tried to resize).

All these variables are in the same scope, never are assigned null which means that they cannot be garbage collected.

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You can increase the maximum heap limit (see Increase heap size in Java), but this will be a serious limitation on your program.

Your program throws out of memory error when writing to ByteArrayOutputStream. This is the 4th time you copy all your data, which means that 3.6GB is already allocated. From this I deduce that your heap is set to 4GB (which is a maximum you can set on 32 bit operating system).

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What you should do is have a loop, read part of the file, encrypt it and write to another file. This will avoid loading entire file into memory. Lines like List bufferedFile = divideArray(plainText, 8192); or resultByteList.add(...) is something that you shouldn't have in your code - you end up storing entire file in memory. The only thing that you need to keep track of is a cursor (i.e. position which says what bytes you already processed), which is O(1) memory complexity. Then you only need as much memory as the chunk your are encoding - which is far smaller than entire file.