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JVM Atlas

Explore the JVM

The machine that runs Java, part by part. Specification: Java SE 25 · Implementation: OpenJDK HotSpot, JDK 25

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PLATE 01The Java Virtual Machine, assembledSPECIFICATIONJava SE 25 · JVMS / JLSIMPLEMENTATIONOpenJDK HotSpot · JDK 25 · x64JVMS conceptHotSpotplatformSCHEMATIC · NOT TO SCALEJAVA PROCESSJAVA VIRTUAL MACHINEone address space: Java heap · Metaspace · code cache · thread stacks · native librariesBefore it runsjavac compiles source code intoclass files.PlatformThe operating system and CPU.java launcherNative librarieslibjava · libnio · libnet · libzip · libnativemath.soClass loadingFinds and prepares Javaclasses.12 parts inside · select to openMemoryStores objects and runtime data.15 parts inside · select to openExecution engineRuns bytecode.9 parts inside · select to openGarbage collectionReclaims memory that is no longer reachable.11 parts inside · select to openNative interfaceConnects Java to nativecode.1 part inside · select to open

Guided tour · 1 of 5

A map of the Java Virtual Machine

Every shape is a real part of the software machine that runs Java programs. The layout is simplified: positions and sizes are schematic, not a picture of memory.

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Accuracy notes for this plate

Knowledge check

Check your understanding of JVM anatomy

8 questions. Answers are explained, right or wrong.

  1. Question 1

    Question 1 · Memory state

    While main runs Person p1 = new Person("Alex");, where is the variable p1 stored?

  2. Question 2

    Question 2 · Predict the output

    What does this line print on JDK 25?

    Predict the outputJava
    System.out.println(String.class.getClassLoader());
  3. Question 3

    Question 3 · Order of events

    Which order does the JVM Specification require for a class to be ready to run its static initializer?

  4. Question 4

    Question 4 · Concept

    Which runtime data area is private to each thread?

  5. Question 5

    Question 5 · Concept

    In HotSpot JDK 25, where is the metadata of a loaded class (its InstanceKlass, methods and bytecode) stored?

  6. Question 6

    Question 6 · Memory state

    After p1 = null; p2 = null;, no reference to the Person remains. What happens to the object immediately?

  7. Question 7

    Question 7 · Concept

    When does HotSpot deoptimize a compiled method?

  8. Question 8

    Question 8 · Predict the output

    On Linux, which file does System.loadLibrary("nativemath") look for on java.library.path?

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JVM Atlas

Explore the machine behind Java programs. A free, open-source atlas of the Java Virtual Machine. Specification baseline: Java SE 25. Implementation details: OpenJDK HotSpot, JDK 25.

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