XMX

Xmx is a Java Virtual Machine option that specifies the maximum size of the Java heap, the memory area used for allocating Java objects. The value is provided as -Xmx followed by a number and a unit, for example -Xmx512m or -Xmx2g. Units commonly used are k, m, and g for kilobytes, megabytes, and gigabytes.

Xmx works together with -Xms, the initial heap size. The JVM starts with the configured -Xms value and can grow the heap up to -Xmx as needed, subject to available system memory. This setting affects only the Java heap; non-heap memory such as metaspace or native allocations, as well as thread stacks, lie outside the heap and are not constrained by -Xmx.

Choosing a value involves trade-offs. A larger -Xmx can reduce the frequency of garbage collection in some workloads but may increase pause times and memory usage, potentially leading to swapping if physical RAM is exhausted. A smaller value reduces memory usage but may cause more frequent garbage collection or out-of-memory errors if the application requires more heap.

System and platform considerations apply. On 32-bit JVMs, the maximum usable heap is limited by address space, typically resulting in lower possible values than on 64-bit JVMs. In containerized environments, memory limits from the host or container can affect usable heap size; modern JVM versions often detect container limits, but misconfiguration can still cause startup or runtime issues.

If the JVM cannot reserve the requested heap, startup may fail with an error such as Could not reserve enough space for object heap. The -Xmx setting is commonly adjusted via startup scripts or environment configuration to fit the application's memory profile.