内存中异常的 Java 大小

Question

Does anyone know how much memory an Exception occupies once it is created and thrown ?

For example, NullPointerException .

And how exceptions are being garbage collected?

Answer 1

Does anyone know how much memory an Exception occupies once it is created and thrown?

That would depend entirely on the exception. Like any other object, it contains variable amount of data; the String message could be 4MB if someone did something silly:

Exception e = 
    new Exception(new String("Some gigantic message ... lalalalalalalalla"));

( Edit: ok, this is somewhat misleading; the exception contains a reference to a String and reference values are a fixed size, but the String itself might only be referenced by the exception - I changed it to be a non-literal to explicitly show it could be something that is collectible. A custom exception could hold anything though, it's an object like any other. In addition, it depends how far it has been thrown, since it holds the stack trace inside of it. There's a good Q/A here on SO; In java, what is the best way to determine the size of an object that covers this. )

And how exceptions are being garbage collected?

Just as any other object is. The Exception is thrown up the call stack and one of two things happen:

1) You catch it, and it's assigned to a variable in the catch block:

catch (Exception e) {

e now holds the one and only reference to the exception. When no more references to it exist (ie it either falls out of scope at the bottom of the catch block, or the object you pass it to gets collected, etc), it will get collected.

2) You don't catch it and it hits the top of the call stack for the current thread. At that point the exception falls out of scope so it will be collected, and the thread is of course halted.

** To be completely pedantic when I say "will get collected" I mean eventually as when an object in Java has no more references to it it's it's eligible for collection, and the GC deals with it.

Answer 2

Java Exceptions are Object so as any Other Objects the size depends by it structure if you have created a custom exception you can (for example) store e complete binary file other Object until you have free memory. You can set the initial and maximum space for your application. The free space change dynamically and now there is the GC question. Java Exceptions are Object so as any Other Objects garbage collection uses the rules in your Enviorment.

For a quick reference about exceptions http://docs.oracle.com/javase/tutorial/essential/exceptions/

this is an article about garbage collection where the key concept are

Summary on Garbage collection in Java

1) Java Heap is divided into three generation for sake of garbage collection. These are young generation, tenured or old generation and Perm area.

2) New objects are created into young generation and subsequently moved to old generation.

3) String pool is created in Perm area of Heap, garbage collection can occur in perm space but depends upon JVM to JVM.

4) Minor garbage collection is used to move object from Eden space to Survivor 1 and Survivor 2 space and Major collection is used to move object from young to tenured generation.

5) Whenever Major garbage collection occurs application threads stops during that period which will reduce application's performance and throughput.

6) There are few performance improvement has been applied in garbage collection in java 6 and we usually use JRE 1.6.20 for running our application.

7) JVM command line options –Xmx and -Xms is used to setup starting and max size for Java Heap. Ideal ratio of this parameter is either 1:1 or 1:1.5 based upon my experience for example you can have either both –Xmx and –Xms as 1GB or –Xms 1.2 GB and 1.8 GB.

8) There is no manual way of doing garbage collection in Java.

Read more: http://javarevisited.blogspot.it/2011/04/garbage-collection-in-java.html

If you use java 7 an intersting question about GC is this

Java 7 (JDK 7) garbage collection and documentation on G1

Other suggestion are useful if you need to view only the state bat if you need to tune the config of your app you need to work with GC alg end memory.

Answer 3

Thats easy to find out by yourself. Launch jvisualvm and attach to the application you want to profile. Switch to the memory tab and filter for the exception you are looking for.

That should give you a good picture of how many bytes are used by exception objects - and also how this relates to your total heap size - and how well and how often they get collected.

Answer 4

Use Java Mission Control for checking this.

Answer 5

I'm wondering why nobody mentioned that the first thing which happens when you create an exception is that the stack trace is populated:

 public Throwable() {
    fillInStackTrace();
 }

If you create your own Exception class and override the fillInStackTrace() method to do nothing, your Exception will become just a simple Java Object and will occupy just as much memory as other objects (which have the same fields/values). But of course you will lose the most precious thing: the stack trace. As a conclusion, most of the memory occupied by the Exception object is due to storing the stack trace (it is stored in an internal format, then converted to StackTraceElement[] when the getStackTrace() method is called). So the larger the stack trace, the more memory the exception will occupy.

public class LightWeightException extends RuntimeException {

    public LightWeightException(String message) {
        super(message);
    }

    @Override
    public synchronized Throwable fillInStackTrace() {
        return this;
    }
}