Understanding Java Heap: Memory Regions, GC Generations, and Allocation Strategies

For Java applications, the Java heap is the largest memory area managed by the JVM, shared by all threads and created at JVM startup to store object instances.

JDK 1.7 Generational Structure

In JDK 1.7 and earlier, the heap is divided into three parts: Young Generation, Old Generation, and Permanent Generation. The Young Generation further consists of Eden and two Survivor spaces.

JDK 1.8 Generational Structure

Starting with JDK 1.8, the Permanent Generation is removed and replaced by Metaspace, which automatically expands. The heap now consists of Young and Old generations without a fixed Permanent Generation.

The runtime data area layout is shown below:

G1 Collector

G1 removes the physical separation of Young and Old generations, dividing the heap into many equal-sized regions. Some regions belong to the Young generation, and G1 still performs copying collection for those regions. The collector also supports Humongous regions for objects larger than 50% of a region size.

Object Memory Allocation

The allocation process is illustrated below.

Objects Preferentially Allocated in Eden

Most objects are allocated in the Eden space of the Young generation. When Eden is full, a Minor GC is triggered. The HotSpot JVM can print detailed GC logs with the -XX:+PrintGCDetails flag.

/**
 * -XX:+PrintGCDetails
 */
public class GCTest {
    public static void main(String[] args) {
        byte[] allocation2 = new byte[8000 * 1024];
    }
}

Sample output of the heap after running the program:

Heap
 PSYoungGen      total 38400K, used 11353K [0x0000000795580000, 0x0000000798000000, 0x00000007c0000000)
   eden space 33280K, 34% used [0x0000000795580000,0x00000007960966f8,0x0000000797600000)
   from space 5120K, 0% used [0x0000000797b00000,0x0000000797b00000,0x0000000798000000)
   to   space 5120K, 0% used [0x0000000797600000,0x0000000797600000,0x0000000797b00000)
 ParOldGen       total 87552K, used 0K [0x0000000740000000, 0x0000000745580000, 0x0000000795580000)
   object space 87552K, 0% used [0x0000000740000000,0x0000000740000000,0x0000000745580000)
 Metaspace       used 3017K, capacity 4556K, committed 4864K, reserved 1056768K
   class space    used 319K, capacity 392K, committed 512K, reserved 1048576K

The output shows that allocation2 was allocated in the Eden space.

Large Objects Directly Enter Old Generation

Objects larger than a threshold set by -XX:PretenureSizeThreshold are allocated directly in the Old generation (effective with Serial and ParNew collectors).

Long‑Lived Objects Move to Old Generation

Objects that survive multiple Minor GCs increase their age counter; once the age reaches the MaxTenuringThreshold (default 15) they are promoted to the Old generation.

Dynamic Object Age Determination

If the total size of objects of a certain age in Survivor exceeds half of the Survivor space, those objects may be promoted early, without waiting for the age threshold.

Space Allocation Guarantee

Before a Minor GC, the JVM checks whether the Old generation has a contiguous free region large enough to accommodate all surviving objects from the Young generation.

Reference Information

《深入理解 JVM 虚拟机-第三版》周志明

https://www.infoq.cn/article/Java-PERMGEN-Removed

https://www.cnblogs.com/wangzhongqiu/p/10250868.html