Why JVM is designed in a way that it does not allow force Garbage Collection?

Question

As far as I know, we can't force for Garbage Collection in JAVA. The best we can do is to send a request by calling System.gc() or Runtime.gc(). Doing so will send request of Garbage collection to JVM but it’s not guaranteed that garbage collection will happen. So my question is: Are there any particular reasons, WHY JVM is designed in a way that it doesn't support Force Garbage Collection?

Answer 1

Forcing garbage collection is inefficient, and unnecessary in most cases¹ ... if you have written your program correctly.

Reference:

• Why is it bad practice to call System.gc()?

It turns out that if you control the launching of the JVM that will run your application (or applet or servlet or whatever) then you can ensure that calling System.gc() will run the GC. Or at least, that is the case with Sun / Oracle JVMs ... where the behaviour is controlled via a -XX option on the java command.

The point that the javadoc is making is that this is platform dependent, and a portable application can't rely on it.

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As to your question:

WHY JVM is designed in a way that it doesn't support Force Garbage Collection?

So that the JVM can be protected against the performance impact of badly written code; e.g. in applets, plugins, 3rd-party libraries, etc.

(And I imagine, in part because the original Sun engineers got a bit fed up with people complaining that "Java is slow" when the real problem was unnecessary calls to System.gc() ...)

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^{1 - But not always. For example, calling System.gc() at a convenient time can be a way to avoid a GC-related pause at an inconvenient time. However, if your code only works if you run the GC at certain points, then you are doing something wrong.}

Answer 2

Java is designed so that (unlike in C++) you do not have to concern yourself with reclaiming disused memory. If the runtime memory is configured correctly, you should not need to concern yourself with garbage collection. Having said that, doing a gc() will generally trigger a sweep of the young generation space.