How do you optimize the heap size usage of an application that has a lot (millions) of long-lived objects? (big cache, loading lots of records from a db)
- Use the right data type
- Avoid java.lang.String to represent other data types
- Avoid duplicated objects
- Use enums if the values are known in advance
- Use object pools
- String.intern() (good idea?)
- Load/keep only the objects you need
I am looking for general programming or Java specific answers. No funky compiler switch.
Edit:
Optimize the memory representation of a POJO that can appear millions of times in the heap.
Use cases
- Load a huge csv file in memory (converted into POJOs)
- Use hibernate to retrieve million of records from a database
Resume of answers:
- Use flyweight pattern
- Copy on write
- Instead of loading 10M objects with 3 properties, is it more efficient to have 3 arrays (or other data structure) of size 10M? (Could be a pain to manipulate data but if you are really short on memory...)
You don't say what sort of objects you're looking to store, so it's a little difficult to offer detailed advice. However some (not exclusive) approaches, in no particular order, are:
- Use a flyweight pattern wherever
possible.
- Caching to disc. There are
numerous cache solutions for
Java.
- There is some debate as to whether
String.intern is a good idea. See
here for a question re.
String.intern(), and the amount of
debate around its suitability.
- Make use of soft or weak
references to store data that you can
recreate/reload on demand. See
here for how to use soft
references with caching techniques.
Knowing more about the internals and lifetime of the objects you're storing would result in a more detailed answer.