# Volatile The Java `volatile` keyword is used to mark a Java variable as "being stored in main memory". More precisely that means, that every read of a volatile variable will be read from the computer's main memory, and not from the CPU cache, and that every write to a volatile variable will be written to main memory, and not just to the CPU cache. Actually, since Java 5 the `volatile` keyword guarantees more than just that volatile variables are written to and read from main memory. I will explain that in the following sections. ### Variable Visibility Problems The Java `volatile` keyword guarantees visibility of changes to variables across threads. This may sound a bit abstract, so let me elaborate. In a multithreaded application where the threads operate on non-volatile variables, each thread may copy variables from main memory into a CPU cache while working on them, for performance reasons. If your computer contains more than one CPU, each thread may run on a different CPU. That means, that each thread may copy the variables into the CPU cache of different CPUs. This is illustrated here:![Threads may hold copies of variables from main memory in CPU caches.](http://tutorials.jenkov.com/images/java-concurrency/java-volatile-1.png) With non-volatile variables there are no guarantees about when the Java Virtual Machine (JVM) reads data from main memory into CPU caches, or writes data from CPU caches to main memory. This can cause several problems which I will explain in the following sections. Imagine a situation in which two or more threads have access to a shared object which contains a counter variable declared like this: ``` public class SharedObject { public int counter = 0; } ``` Imagine too, that only Thread 1 increments the `counter` variable, but both Thread 1 and Thread 2 may read the `counter` variable from time to time. If the `counter` variable is not declared `volatile` there is no guarantee about when the value of the `counter` variable is written from the CPU cache back to main memory. This means, that the `counter`variable value in the CPU cache may not be the same as in main memory. This situation is illustrated here:![The CPU cache used by Thread 1 and main memory contains different values for the counter variable.](http://tutorials.jenkov.com/images/java-concurrency/java-volatile-2.png) The problem with threads not seeing the latest value of a variable because it has not yet been written back to main memory by another thread, is called a "visibility" problem. The updates of one thread are not visible to other threads. ### The Java volatile Visibility Guarantee The Java `volatile` keyword is intended to address variable visibility problems. By declaring the `counter`variable `volatile` all writes to the `counter` variable will be written back to main memory immediately. Also, all reads of the `counter` variable will be read directly from main memory. Here is how the `volatile` declaration of the `counter` variable looks: ``` public class SharedObject { public volatile int counter = 0; } ``` Declaring a variable `volatile` thus guarantees the visibility for other threads of writes to that variable. In the scenario given above, where one thread (T1) modifies the counter, and another thread (T2) reads the counter (but never modifies it), declaring the `counter` variable `volatile` is enough to guarantee visibility for T2 of writes to the `counter` variable. If, however, both T1 and T2 were incrementing the `counter` variable, then declaring the `counter` variable `volatile` would not have been enough. More on that later. #### Full volatile Visibility Guarantee Actually, the visibility guarantee of Java `volatile` goes beyond the `volatile` variable itself. The visibility guarantee is as follows: * If Thread A writes to a `volatile` variable and Thread B subsequently reads the same volatile variable, then all variables visible to Thread A before writing the volatile variable, will also be visible to Thread B after it has read the volatile variable. * If Thread A reads a `volatile` variable, then all all variables visible to Thread A when reading the `volatile` variable will also be re-read from main memory. Let me illustrate that with a code 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; } } ``` The `udpate()` method writes three variables, of which only `days` is volatile. The full `volatile` visibility guarantee means, that when a value is written to `days`, then all variables visible to the thread are also written to main memory. That means, that when a value is written to `days`, the values of `years` and `months` are also written to main memory. When reading the values of `years`, `months` and `days` you could do it like 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; } } ``` Notice the `totalDays()` method starts by reading the value of `days` into the `total` variable. When reading the value of `days`, the values of `months` and `years` are also read into main memory. Therefore you are guaranteed to see the latest values of `days`, `months` and `years` with the above read sequence. ### Instruction Reordering Challenges The Java VM and the CPU are allowed to reorder instructions in the program for performance reasons, as long as the semantic meaning of the instructions remain the same. For instance, look at the following instructions: ``` int a = 1; int b = 2; a++; b++; ``` These instructions could be reordered to the following sequence without losing the semantic meaning of the program: ``` ' int a = 1; a++; int b = 2; b++; ``` However, instruction reordering present a challenge when one of the variables is a `volatile` variable. Let us look at the `MyClass` class from the example earlier in this Java volatile tutorial: ``` 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; } } ``` Once the `update()` method writes a value to `days`, the newly written values to `years` and `months` are also written to main memory. But, what if the Java VM reordered the instructions, like this: ``` public void update(int years, int months, int days){ this.days = days; this.months = months; this.years = years; } ``` The values of `months` and `years` are still written to main memory when the `days` variable is modified, but this time it happens before the new values have been written to `months` and `years`. The new values are thus not properly made visible to other threads. The semantic meaning of the reordered instructions has changed. Java has a solution for this problem, as we will see in the next section. ### The Java volatile Happens-Before Guarantee To address the instruction reordering challenge, the Java `volatile` keyword gives a "happens-before" guarantee, in addition to the visibility guarantee. The happens-before guarantee guarantees that: * Reads from and writes to other variables cannot be reordered to occur after a write to a `volatile`variable, if the reads / writes originally occurred before the write to the `volatile` variable. \ The reads / writes before a write to a `volatile` variable are guaranteed to "happen before" the write to the `volatile` variable. Notice that it is still possible for e.g. reads / writes of other variables located after a write to a `volatile` to be reordered to occur before that write to the `volatile`. Just not the other way around. From after to before is allowed, but from before to after is not allowed. * Reads from and writes to other variables cannot be reordered to occur before a read of a `volatile`variable, if the reads / writes originally occurred after the read of the `volatile` variable. Notice that it is possible for reads of other variables that occur before the read of a `volatile` variable can be reordered to occur after the read of the `volatile`. Just not the other way around. From before to after is allowed, but from after to before is not allowed. The above happens-before guarantee assures that the visibility guarantee of the `volatile` keyword are being enforced. ```java // Rule 1: Variables before volatile cannot come after volatile // Rule 2: Variables after volatile can come before volatile // Original // int i1 // int i2 // volatile int i3 // int i4 // int i5 // Not Possible // int i1 // volatile int i3 // int i2 // int i4 // int i5 // Not Possible // volatile int i3 // int i1 // int i2 // int i4 // int i5 // Possible // int i1 // int i2 // int i4 // int i5 // volatile int i3 ``` ### volatile is Not Always Enough Even if the `volatile` keyword guarantees that all reads of a `volatile` variable are read directly from main memory, and all writes to a `volatile` variable are written directly to main memory, there are still situations where it is not enough to declare a variable `volatile`. In the situation explained earlier where only Thread 1 writes to the shared `counter` variable, declaring the `counter` variable `volatile` is enough to make sure that Thread 2 always sees the latest written value. In fact, multiple threads could even be writing to a shared `volatile` variable, and still have the correct value stored in main memory, if the new value written to the variable does not depend on its previous value. In other words, if a thread writing a value to the shared `volatile` variable does not first need to read its value to figure out its next value. As soon as a thread needs to first read the value of a `volatile` variable, and based on that value generate a new value for the shared `volatile` variable, a `volatile` variable is no longer enough to guarantee correct visibility. The short time gap in between the reading of the `volatile` variable and the writing of its new value, creates an [race condition](http://tutorials.jenkov.com/java-concurrency/race-conditions-and-critical-sections.html) where multiple threads might read the same value of the `volatile` variable, generate a new value for the variable, and when writing the value back to main memory - overwrite each other's values. The situation where multiple threads are incrementing the same counter is exactly such a situation where a `volatile` variable is not enough. The following sections explain this case in more detail. Imagine if Thread 1 reads a shared `counter` variable with the value 0 into its CPU cache, increment it to 1 and not write the changed value back into main memory. Thread 2 could then read the same `counter`variable from main memory where the value of the variable is still 0, into its own CPU cache. Thread 2 could then also increment the counter to 1, and also not write it back to main memory. This situation is illustrated in the diagram below:![Two threads have read a shared counter variable into their local CPU caches and incremented it.](http://tutorials.jenkov.com/images/java-concurrency/java-volatile-3.png) Thread 1 and Thread 2 are now practically out of sync. The real value of the shared `counter` variable should have been 2, but each of the threads has the value 1 for the variable in their CPU caches, and in main memory the value is still 0. It is a mess! Even if the threads eventually write their value for the shared `counter` variable back to main memory, the value will be wrong. ### When is volatile Enough? As I have mentioned earlier, if two threads are both reading and writing to a shared variable, then using the `volatile` keyword for that is not enough. You need to use a [synchronized](http://tutorials.jenkov.com/java-concurrency/synchronized.html) in that case to guarantee that the reading and writing of the variable is atomic. Reading or writing a volatile variable does not block threads reading or writing. For this to happen you must use the `synchronized` keyword around critical sections. As an alternative to a `synchronized` block you could also use one of the many atomic data types found in the [`java.util.concurrent` package](http://tutorials.jenkov.com/java-util-concurrent/index.html). For instance, the [`AtomicLong`](http://tutorials.jenkov.com/java-util-concurrent/atomiclong.html) or [`AtomicReference`](http://tutorials.jenkov.com/java-util-concurrent/atomicreference.html) or one of the others. In case only one thread reads and writes the value of a volatile variable and other threads only read the variable, then the reading threads are guaranteed to see the latest value written to the volatile variable. Without making the variable volatile, this would not be guaranteed. The `volatile` keyword is guaranteed to work on 32 bit and 64 variables. ### Performance Considerations of volatile Reading and writing of volatile variables causes the variable to be read or written to main memory. Reading from and writing to main memory is more expensive than accessing the CPU cache. Accessing volatile variables also prevent instruction reordering which is a normal performance enhancement technique. Thus, you should only use volatile variables when you really need to enforce visibility of variables. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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