Java volatile keyword

xindoo 2021-01-22 12:23:02
java volatile keyword


Java Medium volatile Keywords are used to mark variables as “ Stored in memory ”. Exactly every time volatile Variable read and write are direct operation memory , instead of cpu cache. Actually, ever since java 5 after ,volatile Except for volatile Variables read and write directly out of memory , It's also given more meaning , The article will explain later .

Variable visibility

java volatile Keywords ensure the visibility of variables changing between threads . Sounds like a little bit of leisure , Let me elaborate on .

In multithreaded applications , When thread operation is not volatile variable , Because of performance considerations , Each thread copies a variable from memory to cpu cache in ( translator's note : Reading and writing a disk requires 100ns,level1 cache It only needs 1ns). If your computer has more than one cpu, Each thread runs on a different cpu On , This means that each thread copies the variable to a different cpu cache On , Here's the picture .

For non volatile Variable ,JVM There is no guarantee that every time you write, you read and write from memory , This can lead to a series of problems .

Imagine a scene like this , Multiple threads operate on a variable containing a counter , as follows .

public class SharedObject {
public int counter = 0;
}

If only threads 1 Will increase counter, But threads 1 And thread 2 I read it from time to time counter.

If counter Not declared to be volatile,jvm It's not guaranteed every time counter Write cpu cache Will be synchronized to main memory . That means cpu cache The data in and main memory may not be consistent , As shown in the figure below .

Another thread can't read the latest variable value , Because the data hasn't come from cpu cache Synced to main memory , This is it. Visibility issues , Data updates are not visible to other threads .

Java volatile Visibility assurance

Java volatile Was born to solve the problem of visibility . hold counter Variable declared as volatile, all counter All writes are immediately written to main memory , All reads are read directly from main memory .

volatile Can be used as follows :

public class SharedObject {
public volatile int counter = 0;
}

Declare the variable as volatile Ensure the visibility of other threads to update this variable . In the example above , Threads 1 modify counter Variable , Threads 2 Read only without modification , hold counter Declare as volatile You can guarantee that the thread 2 Read the data correctly . at that time , If the thread 1 Threads 2 Will change this variable , that volatile There is no guarantee of the accuracy of the data , I'll explain it later .

volatile Full visibility guarantees

actually ,Java volatile Visibility guarantees go beyond volatile Variable itself . Visibility is guaranteed as follows . - If the thread A To modify a volatile Variable , And threads B Then I read the same variable , You thread A Writing volatile All variable operations before variables are in thread B Read volatile After changes to threads B It's all visible . - If the thread A Read out volatile Variable , So after it, the thread A All variables will be read again from main memory . The test code is as follows :

public class MyClass {
private int years;
private int months
private volatile int days;
public void update(int years, int months, int days){
this.years = years;
this.months = months;
this.days = days;
}
}

update() Method writes three variables , But only days Be declared volatile. volatile The meaning of full visibility assurance is : When the thread changes days, All variables are synchronized to main memory , ad locum years and months It will also be synchronized to main memory .

In the reading years、months、days When , You can do this .

public class MyClass {
private int years;
private int months
private volatile int days;
public int totalDays() {
int total = this.days;
total += months * 30;
total += years * 365;
return total;
}
public void update(int years, int months, int days){
this.years = years;
this.months = months;
this.days = days;
}
}

When calling totalDays() After the method , When reading days Later on total After the variable ,months and years It's also synchronized from main memory . If in the order above , It's guaranteed that you'll read days,months and years The latest value of .

translator's note : In the above specific reading and writing order , Although only days yes volatile Variable , but days and months It has also been realized. volatile. I guess the reason and cpu Hardware related ,volatile The address to be read before the variable is read is at cpu cache The center is invalid , This ensures that the data must be updated from memory before each read . Similarly, the synchronization will be forced after writing cache Data into main memory , That's it volatile semantics . But actually cpu cache In management cache Data is not in a single address , But with a block In units of , So a block Only one of them volatile Variable , So reading and writing this variable will result in the whole block Synchronize with main memory . in summary , I don't think this part of the original author's blog is referential ,java There's no such commitment , And this kind of visibility estimates and specific cpu To achieve something about , It may not be transferable , It's not recommended to use it like this . So if there are multiple variables that need visibility assurance , We still have to add everything volatile identification .

Instruction reordering challenges

Jvm and cpu It is possible to reorder the largest instruction for performance , But they all guarantee semantic consistency . for example :

int a = 1;
int b = 2;
a++;
b++;

These instructions can be rearranged in the following order with semantic correctness .

int a = 1;
a++;
int b = 2;
b++;

But when a variable is volatile When , Instruction reordering faces a challenge . Let's take another look at the above mentioned MyClass() Example .

public class MyClass {
private int years;
private int months
private volatile int days;
public void update(int years, int months, int days){
this.years = years;
this.months = months;
this.days = days;
}
}

When update() Method write days After the variable ,years and months The latest variables are also written , But if jvm Rearranged the instructions as follows :

public void update(int years, int months, int days){
this.days = days;
this.months = months;
this.years = years;
}

although months and years It will eventually be written to main memory as well , But not in real time , Immediate visibility to other threads cannot be guaranteed . The actual semantics will also change due to instruction reordering . Java In fact, this problem has been solved , Let's go on and see .

Java volatile And order (Happens-Before) Guarantee

To solve the challenge of reordering ,java volatile Keyword visibility also guarantees " Orderliness (happens-before)", The meaning of order assurance is as follows . - Read and write other variables if they are in the original volatile Before variables are written , You can't rearrange to volatile After variables are written . One volatile Read before variable write / Writing guarantees that before writing . Please note that there are special circumstances , for example , Yes volatile Read of other variables after write operation / The write operation will be on volatile Reorder before write operation of . Not vice versa . Back to front is allowed , But it's not allowed from the front to the back . - If you read other variables / Write if you were right in the beginning volatile Variable reading / After writing , It cannot be reordered to volatile Before reading . Please note that , In the reading volatile The reading of other variables before the variable can be done at the time of reading volatile The variables are then reordered . Not vice versa . From front to back is allowed , But back to front is not allowed .

above happens-before It's guaranteed that volatile Keyword strong visibility .

volatile Is not enough

Even though volatile Ensure that data read and write are directly operated from main memory , But there are many more cases volatile Semantics is not enough . In the previous example , Threads 1 Write counter Variable , If you will counter Declare as volatile, Threads 2 Always see the latest values .

But in fact , If multiple threads can write shared volatile Variable and the new value written each time does not depend on the old value , The accuracy of variable values can still be guaranteed , In other words, before a thread writes, it does not need to read it once, and then calculate the next value on the read data .

Reading out - Calculation - In the write mode, the correctness of the values can no longer be guaranteed , Because in the process of calculation , Under this time difference, the data may have been updated by other threads , Multiple threads may compete to write data , There will be data coverage .

So in the example above , Multiple threads update together counter Under the circumstances ,volatile There's no guarantee counter The accuracy of the numerical value has been improved . This situation will be explained in detail below .

Imagine threads 1 Read about counter A variable is 0, And then it added 1, But it didn't go back to main memory , It's written in cpu cache in . At this point the thread 2 Also see counter yes 0, It also adds 1 And only write about your own cpu cache in , Like the picture below .

At this point the thread 1 And thread 2 The data is actually out of sync . We expect counter The actual value should be 2, But in main memory it's 0, In a certain cpu cache Medium is 1. Finally, a thread cpu cache The data in will be synchronized and stored in main memory , But the data is wrong .

When volatile That's enough ?

As mentioned above , If there are multiple threads reading and writing volatile Variable , Only volatile It's not enough , You need to use synchronized To make sure that read and write is an atomic operation . Read and write a volatile Variables don't block other threads , To avoid that , You have to use synchronized key word .

except synchronized outside , You can still use it java.util.concurrent The atomic data types provided in the package , such as AtomicLong perhaps AtomicReferences. If only one thread writes , Other threads are read-only , that volatile That's enough , But don't use volatile Data visibility can't be guaranteed .

Be careful :volatile Ensure that only 32 Bit and 64 Visibility of bit variables .

volatile Performance considerations

volatile It will cause the data read and write to operate on the main memory directly , Read write main memory or read write cpu cache It's much slower .volatile Instruction reordering is also prohibited , Instruction reordering is a common means of performance optimization , So you should only use it when you really need to enforce variable visibility volatile.

Original address

http://tutorials.jenkov.com/java-concurrency/volatile.html

Participation of this paper Tencent cloud media sharing plan , You are welcome to join us , share .

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https://javamana.com/2021/01/20210122115415421a.html

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