![5 Mystic sun.misc.Unsafe
Why it is called Unsafe
#
# A fatal error has been detected by the Java Runtime Environment:
#
# SIGSEGV (0xb) at pc=0x00002ad221034ee5, pid=3680, tid=1091057984
#
# JRE version: Java(TM) SE Runtime Environment (8.0_60-b27) (build 1.8.0_60-b27)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (25.60-b23 mixed mode linux-amd64)
# Problematic frame:
# C [libc.so.6+0x6dee5] cfree+0x25
#
# An error report file with more information is saved as:
# /home/apangin/hs_err_pid3680.log
#](https://image.slidesharecdn.com/unsafe-151018113057-lva1-app6892/85/Do-we-need-Unsafe-in-Java-5-320.jpg)
![18 Inside Unsafe
public native Class<?> defineClass(String name, byte[] b, int off, int len,
ClassLoader loader,
ProtectionDomain protectionDomain);
public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data,
Object[] cpPatches);
public native Object allocateInstance(Class<?> cls) throws InstantiationException;
public native void park(boolean isAbsolute, long time);
public native void unpark(Object thread);
public native int getLoadAverage(double[] loadavg, int nelems);
public native void throwException(Throwable ee);](https://image.slidesharecdn.com/unsafe-151018113057-lva1-app6892/85/Do-we-need-Unsafe-in-Java-18-320.jpg)
![34 Use cases
Coding / Decoding
do {
v1 += getInt(buf, offset) * P2;
v1 = ((v1 << 13) | (v1 >>> 19)) * P1;
v2 += getInt(buf, offset + 4) * P2;
v2 = ((v2 << 13) | (v2 >>> 19)) * P1;
v3 += getInt(buf, offset + 8) * P2;
v3 = ((v3 << 13) | (v3 >>> 19)) * P1;
v4 += getInt(buf, offset + 12) * P2;
v4 = ((v4 << 13) | (v4 >>> 19)) * P1;
} while ((offset += 16) <= limit);
public int getInt(byte[] buf, int off) {
return (buf[off] & 0xff) |
(buf[off + 1] & 0xff) << 8 |
(buf[off + 2] & 0xff) << 16 |
(buf[off + 3]) << 24;
}
public int getInt(byte[] buf, int off) {
return unsafe.getInt(buf,
byteArrayOffset + off);
}
xxHash loop](https://image.slidesharecdn.com/unsafe-151018113057-lva1-app6892/85/Do-we-need-Unsafe-in-Java-34-320.jpg)
![36
Serialization
• Fast conversion to/from byte[]
• Access private fields
- Reflection vs. Unsafe
• Instantiate objects without constructor
- unsafe.allocateInstance()
Use cases](https://image.slidesharecdn.com/unsafe-151018113057-lva1-app6892/85/Do-we-need-Unsafe-in-Java-36-320.jpg)
![47 Replacements
// Desired syntax
VarHandle queueSize = Queue::size;
VarHandle intElementHandle = MethodHandles.lookup()
.arrayElementVarHandle(int[].class);
VarHandle longArrayViewHandle = MethodHandles.lookup()
.arrayElementViewHandle(byte[].class, long[].class, true, false);
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(8);
VarHandle bufferView = MethodHandles.lookup()
.byteBufferViewVarHandle(long[].class, true, false);](https://image.slidesharecdn.com/unsafe-151018113057-lva1-app6892/85/Do-we-need-Unsafe-in-Java-47-320.jpg)
![49 Replacements
// Not in Java 9
MemoryRegion region = MemoryRegion.allocateNative(
"myname", MemoryRegion.UNALIGNED, Long.MAX_VALUE);
VarHandle regionView = region.asVarHandle(long[].class, true, false);
regionView.set(region, 0, Long.MAX_VALUE);
return regionView.get(region, 0);](https://image.slidesharecdn.com/unsafe-151018113057-lva1-app6892/85/Do-we-need-Unsafe-in-Java-49-320.jpg)
![55 Unsafe bugs
Not yet fixed
RandomAccessFile raf = new RandomAccessFile("/dev/shm/cache.tmp", "rw");
raf.setLength(5_000_000_000L);
MappedByteBuffer buf = raf.getChannel().map(READ_WRITE, 0, raf.length());
for (int i = 0; i < 5000; i++) {
buf.put(new byte[1_000_000]);
}
SIGBUS](https://image.slidesharecdn.com/unsafe-151018113057-lva1-app6892/85/Do-we-need-Unsafe-in-Java-55-320.jpg)
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Do we need Unsafe in Java?
- 1. Do we need Unsafe in Java? Andrei Pangin [email protected]
- 3. 3 Can I override object with sun.misc.Unsafe? Is there a way to force unload a class by using the sun.misc.Unsafe class? Can one break a security manager with sun.misc.Unsafe? Rescuing a swallowed Exception in Java How to free memory using Java Unsafe, using a Java reference? Create a mutable java.lang.String Mystic sun.misc.Unsafe
- 4. 4 Mystic sun.misc.Unsafe How to free memory using Java Unsafe, using a Java reference? Player p = (Player) unsafe.allocateInstance(Player.class); // Is there a way of accessing the memory address // from the object reference unsafe.freeMemory(p.hashCode()); http://stackoverflow.com/questions/24429777
- 5. 5 Mystic sun.misc.Unsafe Why it is called Unsafe # # A fatal error has been detected by the Java Runtime Environment: # # SIGSEGV (0xb) at pc=0x00002ad221034ee5, pid=3680, tid=1091057984 # # JRE version: Java(TM) SE Runtime Environment (8.0_60-b27) (build 1.8.0_60-b27) # Java VM: Java HotSpot(TM) 64-Bit Server VM (25.60-b23 mixed mode linux-amd64) # Problematic frame: # C [libc.so.6+0x6dee5] cfree+0x25 # # An error report file with more information is saved as: # /home/apangin/hs_err_pid3680.log #
- 6. 6 What is Unsafe actually • Bridge between Class Library and JVM • For use inside JDK - NIO - AWT - Reflection - java.util.concurrent - MethodHandles • Exists in many JVMs, even on Android Mystic sun.misc.Unsafe
- 7. 7 Mystic sun.misc.Unsafe public final class Unsafe { static { registerNatives(); sun.reflect.Reflection.registerMethodsToFilter(Unsafe.class, "getUnsafe"); } private Unsafe() {} private static final Unsafe theUnsafe = new Unsafe(); @CallerSensitive public static Unsafe getUnsafe() { Class cc = Reflection.getCallerClass(); if (cc.getClassLoader() != null) throw new SecurityException("Unsafe"); return theUnsafe; }
- 8. 8 Mystic sun.misc.Unsafe Getting Unsafe public static Unsafe getUnsafe() throws Exception { Field f = Unsafe.class.getDeclaredField("theUnsafe"); f.setAccessible(true); return (Unsafe) f.get(null); }
- 9. 9 Security hole? Security Manager is off by default $ java -Djava.security.manager java.security.AccessControlException: access denied ("java.lang.RuntimePermission" "accessClassInPackage.sun.misc") Mystic sun.misc.Unsafe
- 10. 10 Mystic sun.misc.Unsafe You’ve been warned! $ javac Test.java Test.java:2: warning: Unsafe is internal proprietary API and may be removed in a future release import sun.misc.Unsafe; ^ $ javac -XDignore.symbol.file Test.java
- 11. Inside Unsafe
- 12. 12 Inside Unsafe public native long allocateMemory(long bytes); public native long reallocateMemory(long address, long bytes); public native void freeMemory(long address); public native byte getByte(long address); public native void putByte(long address, byte x); public native int getInt(Object o, long offset); public native void putInt(Object o, long offset, int x); public native long objectFieldOffset(Field f); public native long staticFieldOffset(Field f); public native int arrayBaseOffset(Class<?> arrayClass); public native void copyMemory(Object srcBase, long srcOffset, Object destBase, long destOffset, long bytes);
- 13. 13 package snow.misc; public class MyUnsafe { public native int getInt(long address); #include <jni.h> JNIEXPORT jint JNICALL Java_snow_misc_MyUnsafe(JNIEnv* env, jobject myUnsafe, jlong address) { return *(jint*)address; } Inside Unsafe
- 14. 14 Inside Unsafe JNI call 1. Create stack frame 2. Move arguments according to ABI 3. Wrap objects into JNI handles 4. Obtain JNIEnv* and jclass 5. Trace method_entry 6. Lock if synchronized 7. Lazy lookup and linking 8. in_java in_native thread transition 9. Call the native function 10. Check for safepoint 11. Switch state to in_java 12. Unlock if synchronized 13. Notify method_exit 14. Unwrap result, reset JNI handles block 15. Handle exceptions 16. Remove stack frame
- 15. 15 Implementation • Most methods are JVM intrinsics - E.g. getInt is a single MOV instruction • Executed in Java or VM context - Beware of mapped I/O (safepoint pauses) Inside Unsafe
- 16. 16 public native int getIntVolatile(Object o, long offset); public native void putIntVolatile(Object o, long offset, int x); public native void putOrderedInt(Object o, long offset, int x); public final native boolean compareAndSwapInt(Object o, long offset, int expected, int x); /* @since 1.8 */ public native void loadFence(); public native void storeFence(); public native void fullFence(); public final int getAndAddInt(Object o, long offset, int delta) { int v; do { v = getIntVolatile(o, offset); } while (!compareAndSwapInt(o, offset, v, v + delta)); return v; } Inside Unsafe
- 17. 17 i.getAndAdd(5) lock addl $0x5,0xc(%r10)loop: mov 0xc(%r10),%eax mov %eax,%r11d add $0x5,%r11d lock cmpxchg %r11d,0xc(%r10) sete %r11b movzbl %r11b,%r11d test %r11d,%r11d je loop JDK 7u72 -XX:+PrintAssembly JDK 8u60 83 46 15 11 132 105 45 43 1 2 3 4 ops/μs threads Inside Unsafe
- 18. 18 Inside Unsafe public native Class<?> defineClass(String name, byte[] b, int off, int len, ClassLoader loader, ProtectionDomain protectionDomain); public native Class<?> defineAnonymousClass(Class<?> hostClass, byte[] data, Object[] cpPatches); public native Object allocateInstance(Class<?> cls) throws InstantiationException; public native void park(boolean isAbsolute, long time); public native void unpark(Object thread); public native int getLoadAverage(double[] loadavg, int nelems); public native void throwException(Throwable ee);
- 19. 19 Inside Unsafe Deprecated in JDK 8, removed in 9 @Deprecated public native void monitorEnter(Object o); @Deprecated public native void monitorExit(Object o); @Deprecated public native boolean tryMonitorEnter(Object o); // Phantom HotSpot intrinsics public native void prefetchRead(Object o, long offset); public native void prefetchWrite(Object o, long offset);
- 20. Use cases
- 21. 21 Off-heap collections • Lists, sets, maps - Large capacity > 2 GB - Predictable GC pauses - No heap fragmentation (CMS Promotion Failures) - Data locality Use cases
- 22. 22 Data locality Key Value Key Value Map.Entry Off-heap layout a b c a b c a b c Use cases
- 23. 23 Use cases Off-heap hash map 0 addr 0 … addr 0 hash time next key (?) value hash time 0 key (?) value entry one.nio.mem.SharedMemoryMap
- 24. 24 OffheapLongList public OffheapLongList(int initialCapacity) { capacity = Math.max(initialCapacity, 4); base = unsafe.allocateMemory(capacity * 8L); } public void add(long value) { if (size >= capacity) { capacity *= 2; base = unsafe.reallocateMemory(base, capacity * 8L); } unsafe.putLong(base + size * 8L, value); size++; } public long get(int index) { if (index < 0 || index >= size) { throw new IndexOutOfBoundsException(); } return unsafe.getLong(base + index * 8L); }
- 25. 25 Use cases When to free memory? class OffheapCleaner<T> extends PhantomReference<T> { static final ReferenceQueue<T> queue = new ReferenceQueue<T>(); final long address; OffheapCleaner(T referent, long address) { super(referent, queue); this.address = address; } } while (true) { OffheapCleaner<T> cleaner = (OffheapCleaner<T>) queue.remove(); unsafe.freeMemory(cleaner.address); }
- 26. 26 Why not ByteBuffers? • Size limit (Integer.MAX_VALUE) • Not thread-safe • No freeMemory public void forceClean(ByteBuffer buffer) { ((sun.nio.ch.DirectBuffer) buffer)).cleaner().clean(); } Use cases
- 27. 27 sun.nio.ch.FileChannelImpl try { // If no exception was thrown from map0, the address is valid addr = map0(imode, mapPosition, mapSize); } catch (OutOfMemoryError x) { // An OutOfMemoryError may indicate that we've exhausted memory // so force gc and re-attempt map System.gc(); try { Thread.sleep(100); } catch (InterruptedException y) { Thread.currentThread().interrupt(); } try { addr = map0(imode, mapPosition, mapSize); } catch (OutOfMemoryError y) { // After a second OOME, fail throw new IOException("Map failed", y); } }
- 28. 28 Cassandra // Hoping GC will remove some not used tables. System.gc(); Thread.sleep(60000); location = estimateFlushPath(); if (location!=null) return new File(location, ssTableFileName).getAbsolutePath(); // Hoping GC will remove some not used tables - sometimes 2 GC cycles are needed. logger_.warn("Still Insufficient disk space to flush "+ssTableFileName); System.gc(); Thread.sleep(60000); location = estimateFlushPath();
- 29. 29 Use cases Direct buffers ↔ Unsafe Field addressField = Buffer.class.getDeclaredField("address"); addressField.setAccessible(true); // ByteBuffer address long address = addressField.getLong(byteBuffer); // address ByteBuffer ByteBuffer result = prototype.duplicate(); addressField.setLong(result, address);
- 30. 30 Use cases Benchmark Benchmark Mode Cnt Score Error Units b.byteBuffer thrpt 15 290,832 ± 7,858 ops/us b.rawMemory thrpt 15 480,799 ± 2,610 ops/us public void byteBuffer() { buf.putLong(0, order.timestamp); buf.put(8, order.operationType); buf.putInt(9, order.productId); buf.putFloat(13, order.price); } public void rawMemory() { unsafe.putLong(addr + 0, order.timestamp); unsafe.put(addr + 8, order.operationType); unsafe.putInt(addr + 9, order.productId); unsafe.putFloat(addr + 13, order.price); }
- 31. 31 I/O and persistence • Persistent caches and storages - Memory-mapped files (64-bit) - Shared memory • Cooperation with OS - Pointer arithmetic, alignment - mlock, madvise etc. Use cases
- 32. 32 Use cases Mapping large files // Mapping Method map0 = FileChannelImpl.class.getDeclaredMethod( "map0", int.class, long.class, long.class); map0.setAccessible(true); long addr = (Long) map0.invoke(f.getChannel(), 1, 0L, f.length()); // Unmapping Method unmap0 = FileChannelImpl.class.getDeclaredMethod( "unmap0", long.class, long.class); unmap0.setAccessible(true); unmap0.invoke(null, addr, length);
- 33. 33 IPC, Messaging • Concurrent off-heap buffers and queues • High-performance messaging - Disruptor - Aeron: 6M msg/s • Shared data structures - Chronicle Map Use cases
- 34. 34 Use cases Coding / Decoding do { v1 += getInt(buf, offset) * P2; v1 = ((v1 << 13) | (v1 >>> 19)) * P1; v2 += getInt(buf, offset + 4) * P2; v2 = ((v2 << 13) | (v2 >>> 19)) * P1; v3 += getInt(buf, offset + 8) * P2; v3 = ((v3 << 13) | (v3 >>> 19)) * P1; v4 += getInt(buf, offset + 12) * P2; v4 = ((v4 << 13) | (v4 >>> 19)) * P1; } while ((offset += 16) <= limit); public int getInt(byte[] buf, int off) { return (buf[off] & 0xff) | (buf[off + 1] & 0xff) << 8 | (buf[off + 2] & 0xff) << 16 | (buf[off + 3]) << 24; } public int getInt(byte[] buf, int off) { return unsafe.getInt(buf, byteArrayOffset + off); } xxHash loop
- 35. 35 Use cases Benchmark Benchmark (length) Mode Cnt Score Error Units b.xxhashArray 10 thrpt 5 61697,875 ± 849,565 ops/ms b.xxhashArray 100 thrpt 5 13552,417 ± 91,039 ops/ms b.xxhashArray 1000 thrpt 5 1749,843 ± 12,290 ops/ms b.xxhashUnsafe 10 thrpt 5 77358,056 ± 675,536 ops/ms b.xxhashUnsafe 100 thrpt 5 34153,796 ± 319,798 ops/ms b.xxhashUnsafe 1000 thrpt 5 5259,813 ± 43,870 ops/ms
- 36. 36 Serialization • Fast conversion to/from byte[] • Access private fields - Reflection vs. Unsafe • Instantiate objects without constructor - unsafe.allocateInstance() Use cases
- 37. 37 one-nio serialization • Extend sun.reflect.MagicAccessorImpl - no bytecode verification • Fast and compact • Supports class evolution - Java serialization - JBoss - Kryo - Protobuf Use cases
- 38. 38 Native APIs • Own network library - Linux-specific TCP features and socket options - Integration with OpenSSL • I/O and memory management - mlock - posix_fadvise • setuid • sched_setaffinity Use cases
- 40. 40 Removal of sun.misc.Unsafe An absolute disaster Cassandra Netty Guava Hazelcast Mockito GWT Hadoop Zookeeper Kafka Gson Neo4j Grails Spring Disruptor JRuby Scala Akka
- 41. 41 Why? Removal of sun.misc.Unsafe Integrity and security Maintenance costs
- 42. 42 Chronology • JEP 200: The Modular JDK • Mark Reinhold speaks at Devoxx • Chris Engelbert started a discussion group • Apocalyptic post at blog.dripstat.com • JCrete • JVM Language Summit • JEP 260: Encapsulate Most Internal APIs Removal of sun.misc.Unsafe 2014 / 07 2015 / 06 2015 / 07 2015 / 08
- 43. 43 Migration plan • Replacement in JDK 8 hide in JDK 9 - sun.misc.BASE64Encoder etc. - Available via command-line flag • No replacement in JDK 8 available outside - sun.misc.Unsafe, sun.reflect.ReflectionFactory - sun.misc.Cleaner, sun.misc.SignalHandler • Replacement in JDK 9 hide in JDK 9, remove in JDK 10 Removal of sun.misc.Unsafe
- 44. Replacements
- 45. 45 Project Panama • JEP 191: Foreign Function Interface • Expected in Java 10 • JNR Replacements public interface LibC { int setuid(@uid_t long uid); } LibC libc = LibraryLoader.create(LibC.class).load("c"); libc.setuid(restrictedUser);
- 46. 46 JEP 193: VarHandles • Targeted for Java 9 Replacements class Queue { int size; ... } VarHandle queueSize = MethodHandles.lookup(). findVarHandle(Queue.class, "size", int.class); queueSize.addAndGet(10);
- 47. 47 Replacements // Desired syntax VarHandle queueSize = Queue::size; VarHandle intElementHandle = MethodHandles.lookup() .arrayElementVarHandle(int[].class); VarHandle longArrayViewHandle = MethodHandles.lookup() .arrayElementViewHandle(byte[].class, long[].class, true, false); ByteBuffer byteBuffer = ByteBuffer.allocateDirect(8); VarHandle bufferView = MethodHandles.lookup() .byteBufferViewVarHandle(long[].class, true, false);
- 48. 48 Replacements public static void fullFence(); public static void acquireFence() public static void releaseFence(); public static void loadLoadFence(); public static void storeStoreFence(); • getRelaxed, getVolatile, getOpaque • setRelaxed, setVolatile, setOpaque • getAquire, setRelease • compareAndSet, compareAndExchangeVolatile • compareAndExchangeAcquire, compareAndExchangeRelease • weakCompareAndSet, weakCompareAndSetAcquire, weakCompareAndSetRelease • getAndSet, getAndAdd, addAndGet
- 49. 49 Replacements // Not in Java 9 MemoryRegion region = MemoryRegion.allocateNative( "myname", MemoryRegion.UNALIGNED, Long.MAX_VALUE); VarHandle regionView = region.asVarHandle(long[].class, true, false); regionView.set(region, 0, Long.MAX_VALUE); return regionView.get(region, 0);
- 50. 50 Replacements Serialization ReflectionFactory reflectionFactory = ReflectionFactory.getReflectionFactory(); Constructor<?> baseConstructor = Object.class.getDeclaredConstructor(); Constructor<?> constructor = reflectionFactory. newConstructorForSerialization(Foo.class, baseConstructor); return (Foo) constructor.newInstance(); jobject AllocObject(JNIEnv *env, jclass clazz); JNI
- 51. 51 Replacements Можно жить и без Unsafe но грустно
- 52. Unsafe bugs
- 53. 53 Unsafe bugs JVM Bugs # # A fatal error has been detected by the Java Runtime Environment: # # SIGSEGV (0xb) at pc=0x00002b9fd9f44898, pid=19692, tid=1096575296 # # JRE version: 6.0_24-b07 # Java VM: Java HotSpot(TM) 64-Bit Server VM (19.1-b02 mixed mode linux-amd64) # Problematic frame: # J Test.loop(Lsun/misc/Unsafe;J)V # # An error report file with more information is saved as: # /home/apangin/hs_err_pid19692.log # # If you would like to submit a bug report, please visit: # http://java.sun.com/webapps/bugreport/crash.jsp # for (int i = 0; i < BUF_SIZE; i += 8) unsafe.putLong(addr + i, Long.reverseBytes(i));
- 54. 54 JVM Bugs • JDK-6968348: Unsafe + Long.reverseBytes • JDK-7196566: Reported by me • JDK-8022783: Non-public bug in C2 • JDK-8087134: Wrong C1 optimization Unsafe bugs 6u25 7u40 7u71 8u60 -XX:-TieredCompilation
- 55. 55 Unsafe bugs Not yet fixed RandomAccessFile raf = new RandomAccessFile("/dev/shm/cache.tmp", "rw"); raf.setLength(5_000_000_000L); MappedByteBuffer buf = raf.getChannel().map(READ_WRITE, 0, raf.length()); for (int i = 0; i < 5000; i++) { buf.put(new byte[1_000_000]); } SIGBUS
- 57. 57 Development @ OK • Blog - http://habrahabr.ru/company/odnoklassniki • Open source - https://github.com/odnoklassniki • Career - http://v.ok.ru Thank you