Can Java write to / read from off heap memory that was freed?

Question

This did surprise me, I'm playing with Java Unsafe. Basically what I am testing is

Allocate unsafe memory -> free the memory -> Write to the freed memory

I was expecting to see some kind of segmentation fault error as I am accessing memory that was freed, but surprisingly, no error / exception was raised.

My code is:

protected static final Unsafe UNSAFE;
static {
    try {
        Field field = Unsafe.class.getDeclaredField("theUnsafe");
        field.setAccessible(true);
        UNSAFE = (Unsafe) field.get(null);
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}
@Test
public void test() {
    long ptr = UNSAFE.allocateMemory(1000);
    UNSAFE.freeMemory(ptr);
    UNSAFE.putOrderedLong(null, ptr, 100L);
}

My question is, if so then why do we need the freeMemory() function in Unsafe? What is it really for?

Answer 1

You only get a signal if you either read addresses which hasn't been assigned a page or write to memory which is read only or not assigned a page.

When you allocate small regions of memory it takes it from a native memory pool which is generally not released back to the operating system when freed.

If you free memory and then allocate it later, that memory can be used for a different purpose. Continuing to use the old address can lead to corruption.