jstat

jstat is a command-line utility bundled with the Java Development Kit (JDK) that provides real-time statistics about a running Java Virtual Machine (JVM). It is designed for lightweight monitoring of memory usage, garbage collection, class loading, and related VM activities with minimal overhead, making it useful for quick diagnostics and tuning tasks. The tool is available on platforms that run the JDK, including Windows, macOS, and Linux.

Overview: jstat connects to a target VM and prints selected statistics to standard output. The target is identified by its process ID (PID). Data can be sampled at regular intervals by supplying an interval and a count. Common options include -class (class loading statistics), -gc (generation and garbage-collection statistics), -gccapacity (memory pool capacities), -gcutil (garbage-collection utilization), and -printcompilation (compilation activity).

Usage: Typical usage is jstat -option pid interval count. For example, jstat -gc 1234 1000 5 prints GC-related statistics for PID 1234 every 1 second, for five samples. Another example, jstat -class 1234 1000 5, shows class-loading activity over the same interval.

Remote monitoring: In environments requiring remote access, a jstatd daemon or the Attach API allows querying statistics from a remote JVM, subject to security policies. Data is obtained from the HotSpot runtime instrumentation.

Limitations: jstat is primarily designed for HotSpot-based VMs and may not expose all metrics on every JVM implementation. Output formats and available columns can vary between JVM versions and builds.