volatile-变量使用指南

Posted on 2023-07-26 Edited on 2024-04-16 In 文献翻译 , java Views:

Guidelines for using volatile variables

volatile 变量使用指南

原文：https://www.ibm.com/developerworks/java/library/j-jtp06197/index.html

参考翻译：https://www.ibm.com/developerworks/cn/java/j-jtp06197.html

Volatile variables in the Java language can be thought of as “synchronized lite”; they require less coding to use than synchronized blocks and often have less runtime overhead, but they can only be used to do a subset of the things that synchronized can. This article presents some patterns for using volatile variables effectively – and some warnings about when not to use them.

Java 语言中的 volatile 变量可以被看作是一种「程度较轻的 synchronized」；与 synchronized 块相比，volatile 变量所需的编码较少，并且运行时开销也较少，但是它所能实现的功能也仅是 synchronized 的一部分。本文介绍几种有效使用 volatile 变量的模式，并强调几种不适合使用 volatile 变量的情形。

Locks offer two primary features: mutual exclusion and visibility. Mutual exclusion means that only one thread at a time may hold a given lock, and this property can be used to implement protocols for coordinating access to shared data such that only one thread at a time will be using the shared data. Visibility is more subtle and has to do with ensuring that changes made to shared data prior to releasing a lock are made visible to another thread that subsequently acquires that lock – without the visibility guarantees provided by synchronization, threads could see stale or inconsistent values for shared variables, which could cause a host of serious problems.

锁提供了两种主要特性：互斥和可见性。互斥一次只允许一个线程持有某个特定的锁，因此可使用该特性实现对共享数据的协调访问协议，一次就只有一个线程能够使用该共享数据。可见性要更加复杂一些，它必须确保释放锁之前对共享数据做出的更改对于随后获得该锁的另一个线程是可见的：如果没有同步机制提供的这种可见性保证，线程看到的共享变量可能是修改前的值或不一致的值，这将引发许多严重问题。

Volatile variables

Volatile 变量

Volatile variables share the visibility features of synchronized, but none of the atomicity features. This means that threads will automatically see the most up-to-date value for volatile variables. They can be used to provide thread safety, but only in a very restricted set of cases: those that do not impose constraints between multiple variables or between a variable’s current value and its future values. So volatile alone is not strong enough to implement a counter, a mutex, or any class that has invariants that relate multiple variables (such as “start <=end”).

Volatile 变量具有 synchronized 的可见性特性，但是不具备原子特性。这就是说线程能够自动发现 volatile 变量的最新值。Volatile 变量可用于提供线程安全，但是只能应用于非常有限的场景：多个变量之间或者某个变量的当前值与修改后值之间没有约束。因此，单独使用 volatile 还不足以实现计数器、互斥锁或任何具有与多个变量相关的不变式（Invariants）的类（例如「start <= end」）。

You might prefer to use volatile variables instead of locks for one of two principal reasons: simplicity or scalability. Some idioms are easier to code and read when they use volatile variables instead of locks. In addition, volatile variables (unlike locks) cannot cause a thread to block, so they are less likely to cause scalability problems. In situations where reads greatly outnumber writes, volatile variables may also provide a performance advantage over locking.

出于简易性或可伸缩性的考虑，你可能倾向于使用 volatile 变量而不是锁。当使用 volatile 变量而非锁时，某些习惯用法（idiom）更加易于编码和阅读。此外，volatile 变量不会像锁那样造成线程阻塞，因此也很少造成可伸缩性问题。在某些情况下，如果读操作远远大于写操作，volatile 变量还可以提供优于锁的性能优势。

Conditions for correct use of volatile

正确使用 volatile 变量的条件

You can use volatile variables instead of locks only under a restricted set of circumstances. Both of the following criteria must be met for volatile variables to provide the desired thread-safety:

你只能在有限的一些情形下使用 volatile 变量替代锁。要使 volatile 变量提供理想的线程安全，必须同时满足下面两个条件：

Writes to the variable do not depend on its current value.

对变量的写操作不依赖于当前值。

The variable does not participate in invariants with other variables.

该变量没有包含在具有其他变量的不变式中。

Basically, these conditions state that the set of valid values that can be written to a volatile variable is independent of any other program state, including the variable’s current state.

实际上，这些条件表明，可以被写入 volatile 变量的这些有效值独立于任何程序的状态，包括变量的当前状态。

The first condition disqualifies volatile variables from being used as thread-safe counters. While the increment operation (x++) may look like a single operation, it is really a compound read-modify-write sequence of operations that must execute atomically – and volatile does not provide the necessary atomicity. Correct operation would require that the value of x stay unchanged for the duration of the operation, which cannot be achieved using volatile variables. (However, if you can arrange that the value is only ever written from a single thread, then you can ignore the first condition.)

第一个条件的限制使 volatile 变量不能用作线程安全计数器。虽然增量操作（x++）看上去类似一个单独操作，实际上它是一个由读取、修改、写入操作序列组成的组合操作，必须以原子方式执行，而 volatile 不能提供必须的原子特性。实现正确的操作需要使 x 的值在操作期间保持不变，而 volatile 变量无法实现这点。（然而，如果将值调整为只从单个线程写入，那么可以忽略第一个条件。）

Most programming situations will fall afoul of either the first or second condition, making volatile variables a less commonly applicable approach to achieving thread-safety than synchronized. Listing 1 shows a non-thread-safe number range class. It contains an invariant – that the lower bound is always less than or equal to the upper bound.

大多数编程情形都会与这两个条件的其中之一冲突，使得 volatile 变量不能像 synchronized 那样普遍适用于实现线程安全。清单 1 显示了一个非线程安全的数值范围类。它包含了一个不变式：下界总是小于或等于上界。

Listing 1. Non-thread-safe number range class

清单 1. 非线程安全的数值范围类

@NotThreadSafe
public class NumberRange {
    private int lower, upper;

    public int getLower() { return lower; }
    public int getUpper() { return upper; }

    public void setLower(int value) {
        if (value > upper)
            throw new IllegalArgumentException(...);
        lower = value;
    }

    public void setUpper(int value) {
        if (value < lower)
            throw new IllegalArgumentException(...);
        upper = value;
    }
}

Because the state variables of the range are constrained in this manner, making the lower and upper fields volatile would not be sufficient to make the class thread-safe; synchronization would still be needed. Otherwise, with some unlucky timing, two threads executing setLower and setUpper with inconsistent values could leave the range in an inconsistent state. For example, if the initial state is (0, 5), and thread A calls setLower(4) at the same time that thread B calls setUpper(3), and the operations are interleaved just wrong, both could pass the checks that are supposed to protect the invariant and end up with the range holding (4, 3) – an invalid value. We need to make the setLower() and setUpper() operations atomic with respect to other operations on the range – and making the fields volatile can’t do this for us.

这种方式限制了范围的状态变量，因此将 lower 和 upper 字段定义为 volatile 类型不能够充分实现类的线程安全；从而仍然需要使用同步。否则，如果凑巧两个线程在同一时间使用不一致的值执行 setLower 和 setUpper 的话，则会使范围处于不一致的状态。例如，如果初始状态是 (0, 5)，同一时间内，线程 A 调用 setLower(4) 并且线程 B 调用 setUpper(3)，显然这两个操作交叉存入的值是不符合条件的，那么两个线程都会通过用于保护不变式的检查，使得最后的范围值是 (4, 3)：一个无效值。至于针对范围的其他操作，我们需要使 setLower() 和 setUpper() 操作原子化：而将字段定义为 volatile 类型是无法实现这一目的的。

Performance considerations

性能考虑

The primary motivation for using volatile variables is simplicity: In some situations, using a volatile variable is just simpler than using the corresponding locking. A secondary motivation for using volatile variables is performance: In some situations, volatile variables may be a better-performing synchronization mechanism than locking.

使用 volatile 变量的主要原因是其简易性：在某些情形下，使用 volatile 变量要比使用相应的锁简单得多。使用 volatile 变量次要原因是其性能：某些情况下，volatile 变量同步机制的性能要优于锁。

It is exceedingly difficult to make accurate, general statements of the form “X is always faster than Y,” especially when it comes to intrinsic JVM operations. (For example, the VM may be able to remove locking entirely in some situations, which makes it hard to talk about the relative cost of volatile vs. synchronized in the abstract.) That said, on most current processor architectures, volatile reads are cheap – nearly as cheap as nonvolatile reads. Volatile writes are considerably more expensive than nonvolatile writes because of the memory fencing required to guarantee visibility but still generally cheaper than lock acquisition.

很难做出准确、全面的评价，例如「X 总是比 Y 快」，尤其是对 JVM 内在的操作而言。（例如，某些情况下 VM 也许能够完全删除锁机制，这使得我们难以抽象地比较 volatile 和 synchronized 的开销。）就是说，在目前大多数的处理器架构上，volatile 读操作开销非常低：几乎和非 volatile 读操作一样。而 volatile 写操作的开销要比非 volatile 写操作多很多，因为要保证可见性需要实现内存界定（Memory Fence），即便如此，volatile 的总开销仍然要比锁获取低。

Unlike locking, volatile operations will never block, so volatiles offer some scalability advantages over locking in the cases where they can be used safely. In cases where reads greatly outnumber writes, volatile variables can often reduce the performance cost of synchronization compared to locking.

volatile 操作不会像锁一样造成阻塞，因此，在能够安全使用 volatile 的情况下，volatile 可以提供一些优于锁的可伸缩特性。如果读操作的次数要远远超过写操作，与锁相比，volatile 变量通常能够减少同步的性能开销。

Patterns for using volatile correctly

正确使用 volatile 的模式

Many concurrency experts tend to guide users away from using volatile variables at all, because they are harder to use correctly than locks. However, some well-defined patterns exist, which, if you follow them carefully, can be used safely in a wide variety of situations. Always keep in mind the rules about the limits of where volatile can be used – only use volatile for state that is truly independent of everything else in your program – and this should keep you from trying to extend these patterns into dangerous territory.

很多并发性专家事实上往往引导用户远离 volatile 变量，因为使用它们要比使用锁更加容易出错。然而，如果谨慎地遵循一些良好定义的模式，就能够在很多场合内安全地使用 volatile 变量。要始终牢记使用 volatile 的限制：只有在状态真正独立于程序内其他内容时才能使用 volatile，这条规则能够避免将这些模式扩展到不安全的用例。

Pattern #1: status flags

模式 #1：状态标志

Perhaps the canonical use of volatile variables is simple boolean status flags, indicating that an important one-time life-cycle event has happened, such as initialization has completed or shutdown has been requested.

也许实现 volatile 变量的规范使用仅仅是使用一个布尔状态标志，用于指示发生了一个重要的一次性事件，例如完成初始化或请求停机。

Many applications include a control construct of the form, “While we’re not ready to shut down, do more work,” as shown in Listing 2:

很多应用程序包含了一种控制结构，形式为「在还没有准备好停止程序时再执行一些工作」，如清单 2 所示：

Listing 2. Using a volatile variable as a status flag

清单 2. 将 volatile 变量作为状态标志使用

volatile boolean shutdownRequested;

……

public void shutdown() { shutdownRequested = true; }

public void doWork() {
    while (!shutdownRequested) {
        // do stuff
    }
}

It is likely that the shutdown() method is going to be called from somewhere outside the loop – in another thread – and as such, some form of synchronization is required to ensure the proper visibility of the shutdownRequested variable. (It might be called from a JMX listener, an action listener in the GUI event thread, through RMI, through a Web service, and so on.) However, coding the loop with synchronized blocks would be much more cumbersome than coding it with a volatile status flag as in Listing 2. Because volatile simplifies the coding, and the status flag does not depend on any other state in the program, this is a good use for volatile.

很可能会从循环外部调用 shutdown() 方法：即在另一个线程中。因此，需要执行某种同步来确保正确实现 shutdownRequested 变量的可见性。（可能会从 JMX 侦听程序、GUI 事件线程中的操作侦听程序、通过 RMI 、通过一个 Web 服务等调用）。然而，使用 synchronized 块编写循环要比使用清单 2 所示的 volatile 状态标志编写麻烦很多。由于 volatile 简化了编码，并且状态标志并不依赖于程序内任何其他状态，因此此处非常适合使用 volatile。

One common characteristic of status flags of this type is that there is typically only one state transition; the shutdownRequested flag goes from false to true and then the program shuts down. This pattern can be extended to state flags that can change back and forth, but only if it is acceptable for a transition cycle (from false to true to false) to go undetected. Otherwise, some sort of atomic state transition mechanism is needed, such as atomic variables.

这种类型的状态标记的一个公共特性是：通常只有一种状态转换；shutdownRequested 标志从 false 转换为 true，然后程序停止。这种模式可以扩展到来回转换的状态标志，但是只有在转换周期不被察觉的情况下才能扩展（从 false 到 true，再转换到 false）。此外，还需要某些原子状态转换机制，例如原子变量。

Pattern #2: one-time safe publication

模式 #2：一次性安全发布

The visibility failures that are possible in the absence of synchronization can get even trickier to reason about when writing to object references instead of primitive values. In the absence of synchronization, it is possible to see an up-to-date value for an object reference that was written by another thread and still see stale values for that object’s state. (This hazard is the root of the problem with the infamous double-checked-locking idiom, where an object reference is read without synchronization, and the risk is that you could see an up-to-date reference but still observe a partially constructed object through that reference.)

缺乏同步会导致无法实现可见性，这使得确定何时写入对象引用而不是原语值变得更加困难。在缺乏同步的情况下，可能会遇到某个对象引用的更新值（由另一个线程写入）和该对象状态的旧值同时存在。（这就是造成著名的双重检查锁定（double-checked-locking）问题的根源，其中对象引用在没有同步的情况下进行读操作，产生的问题是你可能会看到一个更新的引用，但是仍然会通过该引用看到不完全构造的对象）。

One technique for safely publishing an object is to make the object reference volatile. Listing 3 shows an example where during startup, a background thread loads some data from a database. Other code, when it might be able to make use of this data, checks to see if it has been published before trying to use it.

实现安全发布对象的一种技术就是将对象引用定义为 volatile 类型。清单 3 展示了一个示例，其中后台线程在启动阶段从数据库加载一些数据。其他代码在能够利用这些数据时，在使用之前将检查这些数据是否曾经发布过。

Listing 3. Using a volatile variable for safe one-time publication

清单 3. 将 volatile 变量用于一次性安全发布

public class BackgroundFloobleLoader {
    public volatile Flooble theFlooble;

    public void initInBackground() {
        // do lots of stuff
        theFlooble = new Flooble();  // this is the only write to theFlooble
    }
}

public class SomeOtherClass {
    public void doWork() {
        while (true) {
            // do some stuff...
            // use the Flooble, but only if it is ready
            if (floobleLoader.theFlooble != null)
                doSomething(floobleLoader.theFlooble);
        }
    }
}

Without the theFlooble reference being volatile, the code in doWork() would be at risk for seeing a partially constructed Flooble as it dereferences the theFlooble reference.

如果 theFlooble 引用不是 volatile 类型，doWork() 中的代码在解除对 theFlooble 的引用时，将会得到一个不完全构造的 Flooble。

A key requirement for this pattern is that the object being published must either be thread-safe or effectively immutable (effectively immutable means that its state is never modified after its publication). The volatile reference may guarantee the visibility of the object in its as-published form, but if the state of the object is going to change after publication, then additional synchronization is required.

该模式的一个必要条件是：被发布的对象必须是线程安全的，或者是有效的不可变对象（有效不可变意味着对象的状态在发布之后永远不会被修改）。volatile 类型的引用可以确保对象的发布形式的可见性，但是如果对象的状态在发布后将发生更改，那么就需要额外的同步。

Pattern #3: independent observations

模式 #3：独立观察

Another simple pattern for safely using volatile is when observations are periodically “published” for consumption within the program. For example, say there is an environmental sensor that senses the current temperature. A background thread might read this sensor every few seconds and update a volatile variable containing the current temperature. Then, other threads can read this variable knowing that they will always see the most up-to-date value.

安全使用 volatile 的另一种简单模式是：定期「发布」观察结果供程序内部使用。例如，假设有一种环境传感器能够感觉环境温度。一个后台线程可能会每隔几秒读取一次该传感器，并更新包含当前文档的 volatile 变量。然后，其他线程可以读取这个变量，从而随时能够看到最新的温度值。

Another application for this pattern is gathering statistics about the program. Listing 4 shows how an authentication mechanism might remember the name of the last user to have logged on. The lastUser reference will be repeatedly used to publish a value for consumption by the rest of the program.

使用该模式的另一种应用程序就是收集程序的统计信息。清单 4 展示了身份验证机制如何记忆最近一次登录的用户的名字。将反复使用 lastUser 引用来发布值，以供程序的其他部分使用。

Listing 4. Using a volatile variable for multiple publications of independent observations

清单 4. 将 volatile 变量用于多个独立观察结果的发布

public class UserManager {
    public volatile String lastUser;

    public boolean authenticate(String user, String password) {
        boolean valid = passwordIsValid(user, password);
        if (valid) {
            User u = new User();
            activeUsers.add(u);
            lastUser = user;
        }
        return valid;
    }
}

This pattern is an extension of the previous one; a value is being published for use elsewhere within the program, but instead of publication being a one-time event, it is a series of independent events. This pattern requires that the value being published be effectively immutable – that its state not change after publication. Code consuming the value should be aware that it might change at any time.

该模式是前面模式的扩展；将某个值发布以在程序内的其他地方使用，但是与一次性事件的发布不同，这是一系列独立事件。这个模式要求被发布的值是有效不可变的 —— 即值的状态在发布后不会更改。使用该值的代码需要清楚该值可能随时发生变化。

Pattern #4: the “volatile bean” pattern

模式 #4：「volatile bean」模式

The volatile bean pattern is applicable in frameworks that use JavaBeans as “glorified structs.” In the volatile bean pattern, a JavaBean is used as a container for a group of independent properties with getters and/or setters. The rationale for the volatile bean pattern is that many frameworks provide containers for mutable data holders (for instance, HttpSession), but the objects placed in those containers must be thread safe.

volatile bean 模式适用于将 JavaBeans 作为「荣誉结构」使用的框架。在 volatile bean 模式中，JavaBean 被用作一组具有 getter 和/或 setter 方法的独立属性的容器。volatile bean 模式的基本原理是：很多框架为易变数据的持有者（例如 HttpSession）提供了容器，但是放入这些容器中的对象必须是线程安全的。

In the volatile bean pattern, all the data members of the JavaBean are volatile, and the getters and setters must be trivial – they must contain no logic other than getting or setting the appropriate property. Further, for data members that are object references, the referred-to objects must be effectively immutable. (This prohibits having array-valued properties, as when an array reference is declared volatile, only the reference, not the elements themselves, have volatile semantics.) As with any volatile variable, there may be no invariants or constraints involving the properties of the JavaBean. An example of a JavaBean obeying the volatile bean pattern is shown in Listing 5:

在 volatile bean 模式中，JavaBean 的所有数据成员都是 volatile 类型的，并且 getter 和 setter 方法必须非常普通 —— 除了获取或设置相应的属性外，不能包含任何逻辑。此外，对于对象引用的数据成员，引用的对象必须是有效不可变的。（这将禁止具有数组值的属性，因为当数组引用被声明为 volatile 时，只有引用而不是数组本身具有 volatile 语义）。对于任何 volatile 变量，不变式或约束都不能包含 JavaBean 属性。清单 5 中的示例展示了遵守 volatile bean 模式的 JavaBean：

Listing 5. A Person object obeying the volatile bean pattern

清单 5. 遵守 volatile bean 模式的 Person 对象

@ThreadSafe
public class Person {
    private volatile String firstName;
    private volatile String lastName;
    private volatile int age;

    public String getFirstName() { return firstName; }
    public String getLastName() { return lastName; }
    public int getAge() { return age; }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

Advanced patterns for volatile

volatile 的高级模式

The patterns in the previous section cover most of the basic cases where the use of volatile is sensible and straightforward. This section looks at a more advanced pattern where volatile might offer a performance or scalability benefit.

前面几节介绍的模式涵盖了大部分的基本用例，在这些模式中使用 volatile 非常有用并且简单。这一节将介绍一种更加高级的模式，在该模式中，volatile 将提供性能或可伸缩性优势。

The more advanced patterns for using volatile can be extremely fragile. It is critical that your assumptions be carefully documented and these patterns strongly encapsulated because very small changes can break your code! Also, given that the primary motivation for the more advanced volatile use cases is performance, be sure that you actually have a demonstrated need for the purported performance gain before you start applying them. These patterns are trade-offs that give up readability or maintainability in exchange for a possible performance boost – if you don’t need the performance boost (or can’t prove you need it through a rigorous measurement program), then it is probably a bad trade because you’re giving up something of value and getting something of lesser value in return.

volatile 应用的的高级模式非常脆弱。因此，必须对假设的条件仔细证明，并且这些模式被严格地封装了起来，因为即使非常小的更改也会损坏你的代码！同样，使用更高级的 volatile 用例的原因是它能够提升性能，确保在开始应用高级模式之前，真正确定需要实现这种性能获益。需要对这些模式进行权衡，放弃可读性或可维护性来换取可能的性能收益：如果你不需要提升性能（或者不能够通过一个严格的测试程序证明你需要它），那么这很可能是一次糟糕的交易，因为你很可能会得不偿失，换来的东西要比放弃的东西价值更低。

Pattern #5: The cheap read-write lock trick

模式 #5：开销较低的读－写锁策略

By now, it should be well-known that volatile is not strong enough to implement a counter. Because ++x is really shorthand for three operations (read, add, store), with some unlucky timing it is possible for updates to be lost if multiple threads tried to increment a volatile counter at once.

目前为止，你应该了解了 volatile 的功能还不足以实现计数器。因为 ++x 实际上是三种操作（读、添加、存储）的简单组合，如果多个线程凑巧试图同时对 volatile 计数器执行增量操作，那么它的更新值有可能会丢失。

However, if reads greatly outnumber modifications, you can combine intrinsic locking and volatile variables to reduce the cost on the common code path. Listing 6 shows a thread-safe counter that uses synchronized to ensure that the increment operation is atomic and uses volatile to guarantee the visibility of the current result. If updates are infrequent, this approach may perform better as the overhead on the read path is only a volatile read, which is generally cheaper than an uncontended lock acquisition.

然而，如果读操作远远超过写操作，你可以结合使用内部锁和 volatile 变量来减少公共代码路径的开销。清单 6 中显示的线程安全的计数器使用 synchronized 确保增量操作是原子的，并使用 volatile 保证当前结果的可见性。如果更新不频繁的话，该方法可实现更好的性能，因为读路径的开销仅仅涉及 volatile 读操作，这通常要优于一个无竞争的锁获取的开销。

Listing 6. Combining volatile and synchronized to form a “cheap read-write lock”

清单 6. 结合使用 volatile 和 synchronized 实现「开销较低的读写锁」

@ThreadSafe
public class CheesyCounter {
    // Employs the cheap read-write lock trick
    // All mutative operations MUST be done with the 'this' lock held
    @GuardedBy("this") private volatile int value;

    public int getValue() { return value; }

    public synchronized int increment() {
        return value++;
    }
}

The reason this technique is called the “cheap read-write lock” is that you are using different synchronization mechanisms for reads and writes. Because the writes in this case violate the first condition for using volatile, you cannot use volatile to safely implement the counter – you must use locking. However, you can use volatile to ensure the visibility of the current value when reading, so you use locking for all mutative operations and volatile for read-only operations. Where locks only allow one thread to access a value at once, volatile reads allow more than one, so when you use volatile to guard the read code path, you get a higher degree of sharing than you would were you to use locking for all code paths – just like a read-write lock. However, bear in mind the fragility of this pattern: With two competing synchronization mechanisms, this can get very tricky if you branch out beyond the most basic application of this pattern.

之所以将这种技术称之为「开销较低的读－写锁」是因为你使用了不同的同步机制进行读写操作。因为本例中的写操作违反了使用 volatile 的第一个条件，因此不能使用 volatile 安全地实现计数器，你必须使用锁。然而，你可以在读操作中使用 volatile 确保当前值的可见性，因此可以使用锁进行所有变化的操作，使用 volatile 进行只读操作。其中，锁一次只允许一个线程访问值，volatile 允许多个线程执行读操作，因此当使用 volatile 保证读代码路径时，要比使用锁执行全部代码路径获得更高的共享度，就像读写操作一样。然而，要随时牢记这种模式的弱点：如果超越了该模式的最基本应用，结合这两个竞争的同步机制将变得非常困难。

Summary

结束语

Volatile variables are a simpler – but weaker – form of synchronization than locking, which in some cases offers better performance or scalability than intrinsic locking. If you follow the conditions for using volatile safely – that the variable is truly independent of both other variables and its own prior values – you can sometimes simplify code by using volatile instead of synchronized. However, code using volatile is often more fragile than code using locking. The patterns offered here cover the most common cases where volatile is a sensible alternative to synchronized. Following these patterns – taking care not to push them beyond their limits – should help you safely cover the majority of cases where volatile variables are a win.

与锁相比，Volatile 变量是一种非常简单但同时又非常脆弱的同步机制，它在某些情况下将提供优于锁的性能和伸缩性。如果严格遵循 volatile 的使用条件，即变量真正独立于其他变量和自己以前的值，在某些情况下可以使用 volatile 代替 synchronized 来简化代码。然而，使用 volatile 的代码往往比使用锁的代码更加容易出错。本文介绍的模式涵盖了可以使用 volatile 代替 synchronized 的最常见的一些用例。遵循这些模式（注意使用时不要超过各自的限制）可以帮助你安全地实现大多数用例，使用 volatile 变量获得更佳性能。