The document discusses key aspects of how the Java Virtual Machine (JVM) executes Java programs. It describes the launcher, class loading process, bytecode verification, class initialization, just-in-time compilation, threads, synchronization, memory management including garbage collection, and how exceptions and native methods are handled. Troubleshooting techniques like hs_err logs are also mentioned. The JVM performs complex optimizations to efficiently run Java programs.

1. Java Runtime:
повседневные обязанности JVM
Андрей Паньгин
ведущий разработчик
проекта Одноклассники

2. JVM Iceberg
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3. JVM Iceberg
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4. Launcher
• java.exe – just a simple C program
1. Locate JRE
2. Select version
3. Parse arguments
4. JNI_CreateJavaVM
5. JNIEnv::FindClass
6. JNIEnv::GetStaticMethodID (main)
7. JNIEnv::CallStaticVoidMethod
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5. Other launchers
• Differ only by Main Class
– javac com.sun.tools.javac.Main
– javadoc com.sun.tools.javadoc.Main
– javah com.sun.tools.javah.Main
– jconsole sun.tools.jconsole.JConsole
– jmap sun.tools.jmap.JMap
– …
• Even -version calls Java
– sun.misc.Version.print()
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6. Class loading
• Class loading != Class initialization
• ClassLoader prepares byte[] definition
• Real loading is done inside VM
– ClassLoader.defineClass0
– Parses, verifies and creates internal VM structures
• Unreferenced classes may be unloaded
– -XX:+CMSClassUnloadingEnabled
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7. Class Metadata
• Constant Pool, Interfaces, Methods, Fields
• Exceptions, Annotations, vtables, itables
• Java 8: PermGen  Metaspace
– java.lang.OutOfMemoryError: PermGen space
• Field reordering
– -XX:+CompactFields, -XX:FieldsAllocationStyle=1
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8. Class data sharing
• Snapshot of commonly used classes
– jre/lib/classlist
• Mapped into memory directly from disk
– classes.jsa
• Shared between multiple JVM instances
• Improves start-up time, reduces footprint
– -XX:+UseSharedSpaces
– -XX:+DumpSharedSpaces
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9. Bytecode verification
• Java 6 Split Verifier
– Inferencing verifier (javac) + Type checking (run-time)
– StackMapTable attribute in .class
• Skip verification
– -XX:-BytecodeVerificationRemote
– -XX:-BytecodeVerificationLocal
– Inherit sun.reflect.MagicAccessorImpl
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10. Class initialization
• When?
• 12-step procedure described in JLS §12.4
• Basically, the invocation of <clinit>
(static initializer)
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11. Bytecodes
• Stack machine
• 203 Java bytecodes
Stack manipulation iconst, fconst, bipush, pop, dup, swap
Type conversion i2l, f2i, l2d
Arithmetic / logic iadd, isub, imul, ixor, ishr
Local variables iload, aload, istore, fstore
Arrays iaload, aaload, iastore, aastore, arraylength
Fields getfield, putfield, getstatic, putstatic
Branches ifeq, ifgt, goto, tableswitch, lookupswitch
Method calls invokevirtual, invokeinterface, invokestatic, return
Allocation new, anewarray, multianewarray
Other monitorenter, monitorexit, instanceof, athrow
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12. JVM specific bytecodes
• fast_igetfield, fast_iputfield
– -XX:+RewriteBytecodes
• fast_iload2, fast_aaccess_0
– -XX:+RewriteFrequentPairs
• fast_binaryswitch
• return_register_finalizer
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13. Bytecode interpreter
• C++ and Assembler (template) interpreter
• Generated at VM start-up
• Run-time code profiling
– -XX:CompileThreshold=10000
• Run-time CP resolution and bytecode rewriting
• Optimizations
– Dispatch table, top-of-stack caching
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14. Stack
• Java (expression) vs. Native (execution)
– -XX:ThreadStackSize=320
• Guard pages
Empty
– -XX:StackShadowPages=20
– -XX:StackYellowPages=2
Shadow
– -XX:StackRedPages=1
Frame
Frame
Frame
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15. Stack frame SP Expression
stack
Monitors
Method
old SP
FP old FP
Return addr
Locals
Expression
stack
Previous
frame
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16. Frame types
• Interpreted, Compiled, Native
– Different layout
• Inlining
– 1 frame for multiple nested methods
• Deoptimization
– When optimistic assumptions fail
– (exceptions, class hierarchy changes etc.)
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17. Threads
• Java threads & VM threads
• States: in_java, in_vm, in_native, blocked
• Thread pointer in register
– Fast Thread.currentThread()
• Priority policy (0 – normal, 1 – aggressive)
– -XX:ThreadPriorityPolicy=N
• TLAB allocation
– -XX:+UseTLAB, -XX:TLABSize=0
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18. Synchronization
• Simple uncontended lock – CAS
• Biased locking
– -XX:+UseBiasedLocking
– -XX:BiasedLockingStartupDelay=4000
– -XX:BiasedLockingBulkRebiasThreshold=20
– -XX:BiasedLockingBulkRevokeThreshold=40
• Contended lock optimizations
– Spin  Yield  Park
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19. Java-level locks
• java.util.concurrent.locks.LockSupport
• park() / unpark()
• OS-level primitives
– Mutex + Condition variable
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20. wait / notify
• What’s wrong with this code?
void waitForCompletion() { void setCompleted() {
synchronized (lock) { synchronized (lock) {
if (!completed) { completed = true;
lock.wait(); lock.notifyAll();
} }
} }
}
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21. wait / notify
• What’s wrong with this code?
void waitForCompletion() { void setCompleted() {
synchronized (lock) { synchronized (lock) {
if (!completed) { completed = true;
lock.wait(); lock.notifyAll();
} }
} }
}
• -XX:+FilterSpuriousWakeups
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22. Object header
Unlocked
unused hashCode 0 age 0 01
Monitor
Thin lock
Displaced header ptr 00
Inflated lock
Inflated lock ptr 10
Stack
Biased lock
JavaThread epoch 0 age 1 01
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23. Safepoints
• When?
– GC phases
– Thread dump
– Deoptimization
– Revoke/rebias BiasedLock
• How?
– Interpreted: switch dispatch table
– Compiled: page polling
– Native: on return and on JNI calls
• -XX:+PrintSafepointStatistics
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24. Native methods
• System.loadLibrary()
• Lazy linking
• Expensive invocation
1. Create stack frame
2. Set up arguments according to native calling convention
3. Pass JNIEnv* and jclass
4. Lock / unlock if synchronized
5. Trace method entry / method exit
6. Check for safepoint
7. Check exceptions
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25. JNI functions
• Executed in VM context
• Check for safepoint
• jobject == index in thread-local JNIHandles array
• Optional verification
– -XX:+CheckJNICalls
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26. Exceptions
• Which code is faster?
for (;;) { try {
int index = getNextIndex(); for (;;) {
if (index >= arr.length) { int index = getNextIndex();
break; sum += arr[index];
} }
sum += array[index]; } catch (IndexOutOfBoundsException e) {
} // break
}
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27. Exceptions
• Which code is faster?
for (;;) { try {
int index = getNextIndex(); for (;;) {
if (index >= arr.length) { int index = getNextIndex();
break; sum += arr[index];
} }
sum += array[index]; } catch (IndexOutOfBoundsException e) {
} // break
}
• try-catch is free (exception tables)
• throw is expensive
(find handler, unwind stack, release locks, build stack trace)
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28. Reflection
• getDeclaredFields(), getDeclaredMethods()
– VM Internal structures  Java representation
• Field getters and setters
– sun.misc.Unsafe
• Method.invoke()
1. Native implementation
2. Dynamic bytecode generation (up to 10x faster)
-Dsun.reflect.inflationThreshold=15
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29. MethodAccessor example
• void Point.move(float x, float y, boolean relative);
class PointMove_MethodAccessor implements MethodAccessor {
public Object invoke(Object target, Object[] args) {
float x = ((Float) args[0]).floatValue();
float y = ((Float) args[1]).floatValue();
boolean relative = ((Boolean) args[2]).booleanValue();
try {
((Point) target).move(x, y, relative);
} catch (Throwable t) {
throw new InvocationTargetException(t);
}
return null;
}
}
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30. Troubleshooting
• Signal handler (SIGSEGV, SIGILL, SIGFPE…)
• Not all SEGV are fatal
– Polling page
– Implicit NullPointerException, StackOverflowError
• hs_err.log
– Threads, Stack frames, Memory map
– Registers, Top of stack, Instructions
– -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly
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31. Still much to learn
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32. Thank you!
• OpenJDK sources
– http://hg.openjdk.java.net
• Contacts
– andrey.pangin@odnoklassniki.ru
• Open Source @ Odnoklassniki
– https://github.com/odnoklassniki
• Career @ Odnoklassniki
– http://v.ok.ru
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