What are common JVM flags?

JVM flags are used to configure Java virtual machine behavior, mainly covering memory settings, garbage collector selection and debugging diagnosis. In terms of memory, -Xms sets the initial heap size, -Xmx sets the maximum heap size, -Xmn sets the young generation size, and reasonable configuration can avoid frequent GC or OOM. In terms of garbage collectors, UseSerialGC is suitable for small applications, UseParallelGC is suitable for throughput priority services, and UseG1GC is suitable for modern low-latency scenarios. Debugging related parameters such as PrintGCDetails printing GC logs, Xloggc output logs to files, HeapDumpOnOutOfMemoryError when generating OOM, agentlib or javaagent connection performance tools, which helps to analyze the problem. Mastering these commonly used flags can significantly improve the stability and performance of Java applications.

JVM flags (Java virtual machine parameters) are important options for configuring JVM behavior when starting Java programs. They can affect key aspects such as memory settings, garbage collection strategies, and performance tuning. For development and operation and maintenance personnel, it is very practical to master some commonly used JVM flags.

Memory-related configuration

The default memory configuration of JVM often cannot meet the needs of actual applications, especially for scenarios where high concurrency or big data processing are performed. You can adjust the heap memory size by following common parameters:

• -Xms : Set the initial heap size of the JVM. For example, -Xms512m means that the initial heap is 512MB.
• -Xmx : Set the maximum heap size of JVM. For example, -Xmx2g means that the maximum heap is 2GB.
• -Xmn : Set the size of the young generation. Appropriately increasing it can improve the allocation efficiency of object, but it will reduce the space of the elderly.

It is recommended to set these values ​​reasonably according to the application load to avoid frequent GC or OOM (Out Of Memory) errors. Usually, setting -Xms and -Xmx to the same value can reduce the overhead of dynamic adjustment.

Garbage collector selection

Different GC policies are suitable for different types of Java applications. Common GC flags are as follows:

• -XX: UseSerialGC : Use serial GC, suitable for small applications or single-core machines.
• -XX: UseParallelGC : ParallelGC, suitable for throughput-first services.
• -XX: UseConcMarkSweepGC : CMS GC, focusing on low latency, but has been marked as abandoned after Java 8.
• -XX: UseG1GC : G1 GC, a modern mainstream GC method, balancing throughput and latency.

If your application is sensitive to response time, such as web services, it is recommended to use G1GC; while batch tasks focus more on throughput, you can choose ParallelGC.

Debugging and diagnostic related flags

Debug class parameters can help you analyze the running JVM status:

• -XX: PrintGCDetails and -XX: PrintGCDateStamps : Print detailed GC logs for easy subsequent analysis.
• -Xloggc:<file-path></file-path> : Output the GC log to the specified file, for example -Xloggc:/var/log/app-gc.log .
• -XX: HeapDumpOnOutOfMemoryError : Generates a heap dump file when OOM occurs, for subsequent analysis using tools (such as MAT).
• -agentlib:jprofilerti or -javaagent:/path/to/your-agent.jar : Used to connect to performance analysis tools such as JProfiler or VisualVM.

This type of parameters is very useful when there is a problem with the production environment, but some will affect performance, so it is recommended to enable it only if needed.

Basically that's it. There are many types of JVM flags, but the above are the most common and practical parts. Understanding and rational use of them can significantly improve the stability and performance of Java applications.

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