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This article introduces the relevant knowledge of "what Unsafe classes are there". In the operation of actual cases, many people will encounter such a dilemma. Next, let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

1. Brief introduction

First of all, the source code of the sun.misc package cannot be obtained in the Jdk8 of Oracle. If you want to see the source code of this package, you can download openjdk directly.

1. Preparatory work

The source code of openjdk I downloaded is openjdk-8u40-src-b25-10_feb_2015, if you need it, you can trust me privately. If you are a fan of my official account, I will directly attach this Baidu Cloud resource. After the download is complete, you can then import our eclipse directly.

Windows- > preference- > installed jres- > Select jre- > edit- > rt.jar- > source attachment- > external folders- > openjdk source code path. At this point, you can view the source code of our Unsafe class.

2. Brief description

If you have learned some of the class source code in the java concurrency package, you must be familiar with this Unsafe class. It is the core of the underlying implementation of the entire java concurrency package. It was widely rumored a long time ago that this class will be removed from jdk9. In fact, if you remove a large number of frameworks, such as the famous Netty framework. Finally, when jdk9 appeared, it was only improved and optimized. But this once again illustrates the importance of this category.

Why is it half an angel and half a devil? To answer this question, we still have to explain it in terms of its characteristics.

The Unsafe class gives Java the ability to manipulate memory space like pointers in C, which means that (1) it is not managed by jvm, which means it cannot be GC, which requires us to manually GC, and memory leaks will occur at the slightest inadvertent.

(2) in many methods of Unsafe, the original address (memory address) and the address of the object to be replaced must be provided, and the offset must be calculated by yourself. Once a problem occurs, the exception at the level of JVM crash will cause the entire JVM instance to crash, as shown by the direct crash of the application.

(3) the direct operation of memory also means that it is faster and can improve efficiency under the condition of high concurrency.

Therefore, from the above three angles, although it improves the efficiency to a certain extent, it also brings the insecurity of the pointer.

Let's dive into the source code to see what methods are provided to directly manipulate memory.

Second, source code analysis

There are a total of 82 public native decorated methods in Unsafe, and dozens of other methods based on these 82 public native methods. These methods can be summarized into the following categories:

(1) initialization operation

(2) Operand attributes

(3) Operand array elements

(4) memory management

(5) memory barrier

(6) Thread suspend and reply

(7) CAS mechanism

Let's make a general analysis of these methods.

1. Initialize

/ / 1. Register native method. Yes, Unsafe class can operate C language private static native void registerNatives (); static {registerNatives (); sun.reflect.Reflection.registerMethodsToFilter (Unsafe.class, "getUnsafe");} / / 2, constructor private Unsafe () {} / / 3, initialization method private static final Unsafe theUnsafe = new Unsafe () / / 4. Initialization method implements @ CallerSensitive public static Unsafe getUnsafe () {Class caller = Reflection.getCallerClass (); if (! VM.isSystemDomainLoader (caller.getClassLoader () throw new SecurityException ("Unsafe"); return theUnsafe;}

Here we see that the initialization method of Unsafe is mainly implemented through the singleton pattern of the getUnsafe method, calling the JVM local methods registerNatives () and sun.reflect.Reflection, and determining whether the currently called class is loaded by the main class loader (BootStrap classLoader) through the getCallerClass of Reflection, otherwise a SecurityException is thrown. This also proves the problem that only classes loaded by the main class loader (BootStrap classLoader) can call methods in this class.

2. Operation attribute method

(1) public native Object getObject (Object o, long offset)

Gets the reference value through the given Java variable. This is actually getting the value of an attribute with an offset address of offset in a Java object. This method breaks through the suppression of modifiers, that is, ignoring the private, protected, and default modifiers. Similar methods include getInt, getDouble, and so on. Similarly, there is the putObject method.

(2) public native Object getObjectVolatile (Object o, long offset)

Forces the property value to be obtained from main memory. Similar methods include getIntVolatile, getDoubleVolatile, and so on. Similarly, there is putObjectVolatile.

(3) public native void putOrderedObject (Object o, long offset, Object x)

Set the value of the object field corresponding to the offset offset address offset in the o object to the specified value x. This is an ordered or delayed putObjectVolatile method, and changes in unguaranteed values are immediately seen by other threads. Use will only take effect if the field is modified by volatile and is expected to be modified. Similar methods are putOrderedInt and putOrderedLong.

(4) public native long staticFieldOffset (Field f)

Returns the location (offset address) of the given static property in the storage allocation of its class.

(5) public native long objectFieldOffset (Field f)

Returns the location (offset address) of a given non-static property in the storage allocation of its class.

(6) public native Object staticFieldBase (Field f)

Returns the location of the given static property, used with the staticFieldOffset method.

3. Operand array

(1) public native int arrayBaseOffset (Class arrayClass)

Returns the offset address (base offset address) of the first element of the array type.

(2) public native int arrayIndexScale (Class arrayClass)

Returns the increment of the offset address between elements in the array.

These two methods work together to locate the address of any element.

4. Memory management

(1) public native int addressSize ()

Gets the size of the local pointer in byte, with a normal value of 4 or 8. The constant ADDRESS_SIZE calls this method.

(2) public native int pageSize ()

Gets the number of pages in local memory, which is the power of 2.

(3) public native long allocateMemory (long bytes)

Allocate a new piece of local memory, specify the size of the memory block (in byte) through bytes, and return the address of the newly opened memory.

(4) public native long reallocateMemory (long address, long bytes)

Resize the local memory block through the specified memory address address, and the resized memory block is specified by bytes (in byte).

(5) public native void setMemory (Object o, long offset, long bytes, byte value)

Sets all bytes in a given memory block to a fixed value (usually 0).

5. Thread suspension and recovery

(1) public native void unpark (Object thread)

Releases blocking on a thread created by park. Because it is not safe, you must ensure that the thread is alive.

(2) public native void park (boolean isAbsolute, long time); `

Block the current thread, waiting for the unpark method to be called.

6. Memory barrier

(1) public native void loadFence ()

All reads prior to this method must be performed before the load barrier.

(2) public native void storeFence ()

All writes before this method must be completed before the store barrier

(3) public native void fullFence ()

All read and write operations prior to this method must be performed before the full barrier, which is equivalent to the combined function of the above two (load barrier and store barrier).

7. CAS mechanism

Public final native boolean compareAndSwapObject (Object o, long offset, Object expected, Object x); public final native boolean compareAndSwapInt (Object o, long offset,int expected, int x); public final native boolean compareAndSwapLong (Object o, long offset, long expected,long x)

This Unsafe class actually runs through the whole java concurrent package system. Whether you look at the atomic package or the bottom of the lock package, there is such a class. What we need to remember is not every method, but the title of the above seven categories. That is, what function does it have?

III. Use

The source code that has been talked about for so long is introduced here because we are not recommended to use it, that is to say, we cannot directly new a Unsafe class, so how should we use it? I wrote an article about the java reflection mechanism a long time ago, and that's right. It's not awesome. Unsafe can be obtained through the reflection mechanism.

Public class UnsafeTest {public static void main (String [] args) throws Exception {/ / the theUnsafe here is the theUnsafe Field theUnsafe = Unsafe.class.getDeclaredField ("theUnsafe") in our source code; theUnsafe.setAccessible (true); Unsafe unsafe = (Unsafe) theUnsafe.get (null); / / 1. Create object instance Author author = (Author) unsafe.allocateInstance (Author.class) / / 2. Field ageField = Author.class.getDeclaredField ("age"); long fieldOffset = unsafe.objectFieldOffset (ageField); / / 3. Operand array String [] strings = new String [] {"1", "2", "3"}; long I = unsafe.arrayBaseOffset (String [] .class) / / 4. Operating memory long address = unsafe.allocateMemory (8L);}}

Here are just some simple examples, whose usage can refer to the seven directions in the source code analysis.

Note: there is a tip during the interview. For java language features, there are many functions that violate the java language design can be answered with this class.

This is the end of the content of "what are the Unsafe classes"? thank you for reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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