Java中未回收的垃圾内存是否会被计入Runtime计算的已用空间？

Awesome questions—let me break this down for you clearly, since this is a common point of confusion when working with Java memory:

Yes, it does—both from the JVM's internal perspective and how OS-level tools (like top on Linux or Task Manager on Windows) report memory usage. Here's the breakdown:

• When the JVM starts, it requests a block of memory (the heap) from the operating system. Any object that's created but later becomes unreachable (i.e., garbage) still occupies space in this heap until the garbage collector (GC) runs and cleans it up.
• From the OS's view, all memory the JVM has requested is considered "used" by the Java process. The OS doesn't understand the JVM's internal garbage classification—it just sees the total memory the JVM has reserved for its heap.

Definitely. Let's unpack what those methods actually measure:

• rt.totalMemory(): Returns the total amount of memory the JVM has already allocated from the OS for its heap (this isn't the maximum heap size, just what's currently in use by the JVM).
• rt.freeMemory(): Returns the portion of the heap that's currently unused—this includes space that was never allocated to objects, plus space that was freed after the GC collected unreachable objects.
• When you subtract the two, the result is the total memory occupied by all objects in the heap—including those that are no longer reachable (garbage) but haven't been collected yet.

Quick tip for more accurate "live" memory measurement #

If you want to calculate the memory used only by reachable (live) objects, you can hint to the JVM to run garbage collection first (note: System.gc() is a hint, not a guarantee the GC will run immediately) before doing the calculation:

System.gc(); // Suggest to JVM to perform garbage collection
Runtime rt = Runtime.getRuntime();
long usedLiveMemory = rt.totalMemory() - rt.freeMemory();

Even after GC, you might still see some residual memory usage—this could be from soft references, cached objects, or memory the JVM keeps reserved to avoid frequent OS memory requests.

内容的提问来源于stack exchange，提问作者D. O.