Is it possbile to see how much heap an object is using?

If I have a List<Obj开发者_Go百科ect>, would it be possible to run some method on each Object to see how much memory each is consuming? I know nothing about each Object it may be an entire video file loaded onto the heap or just a two-byte string. I ultimately would like to know which objects to drop first before running out of memory.

I think Runtime.totalMemory() shows the memory currently used by the JVM, but I want to see the memory used by a single object.

---

SoftReference looks kinda like what you need. Create a list of soft references to your objects, and if those objects are not referenced anywhere and you run out of memory, JVM will delete some of them. I don't know how smart the algorithm for choosing what to delete is, but it could as well be removing those that will free most memory.

---

If you are in a container you can use Jconsole http://java.sun.com/developer/technicalArticles/J2SE/jconsole.html

The jdk since 1.5 comes with heap dump ulits... You in a container or in eclipse? Also why do you have a List of Objects??

---

There is no clean way to do it. You can create a dummy OutputStream which will do nothing but counting number of bytes written. So, you can make some estimation about your object graph size by serializing it to such stream.

I would not advise to do it in production system. I, personally, did it once for experimenting and making estimations.

---

Actually another possible tactic is just to make a crap load of instance of the class you want to check (like a million in an array).

The sheer number of objects should negate the overhead (as in the overhead of other stuff will be much smaller than your crap load of objects).

You will want to run this in isolation of course (ie public static main()).

I will admit you will need lots of memory for this test.

---

Something you could do is make a Map<Object, Long> which maps each object to it's memory size.

Then to measure the size of a particular object, you have to do it at instantiation of each object - measure the JVM memory use before (calling Runtime.totalMemory()) and after building the object (calling Runtime.totalMemory()) and take the difference between the two - that is the size of the object in memory. Then add the Object and Long to your map. From there you should be able to loop through all of the keys in the map and find the object using the largest amount of space.

I am not sure there is a way to do it per object after you already have your List<Object>... I hope this is helpful!