Do we need Unsafe in Java?

Do we need Unsafe in Java?
Andrei Pangin
andrey.pangin@corp.mail.ru

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 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

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
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

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;
}

Getting Unsafe
public static Unsafe getUnsafe() throws Exception {
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
return (Unsafe) f.get(null);
}

9
Security hole?
Security Manager is off by default
$ java -Djava.security.manager
java.security.AccessControlException:
access denied
("java.lang.RuntimePermission"
"accessClassInPackage.sun.misc")

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

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
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 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
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
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
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 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 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);

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
Data locality
Key
Value
Key
Value
Map.Entry Off-heap layout
a b c a b c a b c
Use cases

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 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 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
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 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 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 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 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
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 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
IPC, Messaging
• Concurrent off-heap buffers and queues
• High-performance messaging
- Disruptor
- Aeron: 6M msg/s
• Shared data structures
- Chronicle Map
Use cases

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 = ((v3 << 13) | (v3 >>> 19)) * P1;
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 Use cases
Benchmark
Benchmark (length) Mode Cnt Score Error Units
b.xxhashArray 10 thrpt 5 61697,875 ± 849,565 ops/ms
b.xxhashUnsafe 10 thrpt 5 77358,056 ± 675,536 ops/ms

36
Serialization
• Fast conversion to/from byte[]
• Access private fields
- Reflection vs. Unsafe
• Instantiate objects without constructor
- unsafe.allocateInstance()
Use cases

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
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 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
Why?
Removal of sun.misc.Unsafe
Integrity and security
Maintenance costs

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
2014 / 07
2015 / 06
2015 / 07
2015 / 08

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

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
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 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 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 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 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 Replacements
Можно жить и без Unsafe
но грустно

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
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 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
Development @ OK
• Blog
- http://habrahabr.ru/company/odnoklassniki
• Open source
- https://github.com/odnoklassniki
• Career
- http://v.ok.ru
Thank you

Do we need Unsafe in Java?

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