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Introduction to Unsafe

Unsafe is a class under the sun.misc package, which mainly provides some methods for performing low-level and unsafe operations, such as directly accessing system memory resources, managing memory resources independently, and so on. These methods play an important role in improving the running efficiency of Java and enhancing the operation ability of the underlying resources of Java language.

However, the Unsafe class makes the Java language have the ability to manipulate memory space like C language pointers, which undoubtedly increases the risk of pointer problems in the program.

Excessive and incorrect use of Unsafe classes in programs will increase the probability of program errors, making Java, a secure language, no longer "safe", so we must be cautious in using Unsafe.

Get Unsafe instance private static sun.misc.Unsafe getUnsafe () {try {return AccessController.doPrivileged (new PrivilegedExceptionAction () {@ Override public sun.misc.Unsafe run () throws Exception {Class k = sun.misc.Unsafe.class; for (Field f: k.getDeclaredFields ()) {f.setAccessible (true)) Object x = f.get (null); if (k.isInstance (x)) {return k.cast (x);}} / / The sun.misc.Unsafe field does not exist. Throw new Error ("unsafe is null");}} catch (Throwable e) {throw new Error ("get unsafe failed", e);}} Unsafe feature list

AllocateMemory/freeMemory, allocates and releases out-of-heap memory DirectMemory (same as malloc in c/cpp)

CAS operation

CopyMemory

DefineClass (without security checks)

Get/put address uses out-of-heap memory addresses for data read and write operations

Get/put volatile uses out-of-heap memory addresses for data read and write operations-volatile version

LoadFence/storeFence/fullFence prohibits instruction reordering

Park/unpark blocking / unblocking threads

Array operation of Unsafe

In unsafe, there are two methods for arrays:

Public native int arrayBaseOffset (Class arrayClass); public native int arrayIndexScale (Class arrayClass); base offset meaning

First of all, in Java, arrays are also objects

In the Java programming language, arrays are objects (§4.3.1), are dynamically created, and may be assigned to variables of type Object (§4.3.2). All methods of class Object may be invoked on an array.

Https://docs.oracle.com/javase/specs/jls/se7/html/jls-10.html

So since it is an object, there will be an object header that takes up part of the space, so it is not difficult to understand the base offset of the array.

For example, in the following JOL output, the attribute data of the object actually starts at the location of OFFSET 16, and 0-12 of the space is occupied by header.

HotSpot 64-bit VM, COOPS 8-byte alignmentlambda.Book object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 12 (object header) N String Book.title A 12 4 int Book.sales N hand A 16 4 String Book.title List Book.tags N/AInstance size: 32 bytesSpace losses: 0 bytes internal + 0 bytes external = 0 bytes total

Then if you want to access the attribute data of an object, you need to offset based on the base address (base address), and the base address + base offset (base offset) + attribute offset (field offset) is the memory address (logic) of the data, then the memory address of the title attribute is actually:

Book (instance) .title field address = book object base address + base offset + title field offset

Arrays can be regarded as a special kind of object in Java. No matter what type of array they are, they will have a basic offset in memory, similar to object header.

After testing, the base offset of 64-bit JVM array types is 16 (test results may vary from JVM to JVM)

The base offset of the original type is 12 (test results may vary from JVM to JVM)

You can use the JOL tool to check the offset of the original type wrapper class. For example, int looks at Integer. Although jdk does not provide a function, it can also be obtained through JOL. The jvm type can be matched with its method.

Index scale meaning

The amount of space occupied by each element in the index group. For example, int [] scale is 4 MagneLong [] scale is 8 Magi object [] scale is 4 (pointer size)

With this offset, you can copyMemory the array.

Public native void copyMemory (Object srcBase, long srcOffset, Object destBase, long destOffset, long bytes); array copy to direct memory

When using copyMemory operation, you need to pass in the object and the base offset of the object. For arrays, offset is the offset described above, for example, now copy the data from an array to DirectBuffer

Byte [] byte = new byte [4096]; unsafe.copyMemory (byte,ARRAY_BYTE_BASE_OFFSET,null,directAddr,4096)

The reason for using unsafe instead of ByteBuffer to manipulate DirectBuffer is that ByteBuffer is inflexible.

For example, if I want to copy a byte [] to a location in DirectBuffer, there is no corresponding method; I can only set position first, and then put (byte [], int, int), which is very troublesome, and position (long) and put are also non-atomic operations when accessing concurrently.

But it's easy to operate with unsafe. Just copyMemory directly and specify address + offset directly.

Unsafe.copyMemory (byte,ARRAY_BYTE_BASE_OFFSET,null,directAddr,4096); copyMemory (Object srcBase, long srcOffset, Object destBase, long destOffset, long bytes)

SrcBase raw data object, which can be an object, an array (object), or a Null (offset,offset must be specified as address when it is Null)

The base offset of the original data object of srcOffset. This item is address if srcBase is empty.

DestBase target data object, the rules are the same as src

The base offset of the destOffset target data object, with the same rules as src

Size of data to be copied by bytes (in bytes)

With the copyMemory method, you can do various copy operations:

Object (usually an array) is copied to the specified out-of-heap memory address

Long l = unsafe.allocateMemory (1); data2 [0] = 5 / goal is memory address,destBase is null,destOffset is addressunsafe.copyMemory (data2,16,null,l,1)

Copy an object to an object

Byte [] data1 = new byte [1]; data1 [0] = 9 byte [] data2 = new byte [1]; unsafe.copyMemory (data1,16,data2,16,1)

Copy data from an out-of-heap memory address to the heap (usually an array)

Byte [] data2 = new byte [1]; long l = unsafe.allocateMemory (1); unsafe.putByte (l, (byte) 2); / / Source data is memory address,srcBase, null,srcOffset is addressunsafe.copyMemory (null,l,data2,16,1)

Out-of-heap memory addresses copy each other

Long l = unsafe.allocateMemory (1); long L2 = unsafe.allocateMemory (1); unsafe.putByte (l, (byte) 2); / / Source data is memory address,srcBase for null,srcOffset, address// target is memory address,destBase, null,destOffset is addressunsafe.copyMemory (null,l,null,l2,1); Benchmark

Sun.misc.Unsafe#putInt (java.lang.Object, long, int) & object field manual set

Disable JIT result:

BenchmarkModeCntScoreErrorUnitsObjectFieldSetBenchmark.manualSetthrpt28646455.472ops/nsObjectFieldSetBenchmark.unsafeSetthrpt27901066.170ops/ns

Enable JIT results:

BenchmarkModeCntScoreErrorUnitsObjectFieldSetBenchmark.manualSetthrpt2477232013.545ops/nsObjectFieldSetBenchmark.unsafeSetthrpt2499135982.962ops/ns

The conclusion is almost the same.

When to use Unsafe

Generally, when using DirectBuffer, you need to use it with Unsafe. Because the memory of DirectBuffer is allocated outside the JVM Heap and belongs to C Heap, you need to directly manipulate the memory address (logic), the same way you operate after malloc in C.

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