Java 17更新

strictfp的用例集非常狭窄，从Java 17开始，它的功能已经被删除了。它仍然是一个有效的修饰符，但是现在strictfp什么都不做(JLS来源)。

相反，现在所有浮点操作都是严格的，就像在Java 1.2中引入strictfp之前一样。在现代处理器上，不再有任何额外的性能成本。

原来的答案

以下是一些参考资料:

Using strictfp (JDC Tech Tip) jGuru: What is the strictfp modifier for? When would I consider using it? Basically, what it all boils down to is whether or not you care that the results of floating-point expressions in your code are fast or predictable. For example, if you need the answers that your code comes up with which uses floating-point values to be consistent across multiple platforms then use strictfp. strictfp - Java Glossary Floating point hardware calculates with more precision, and with a greater range of values than the Java specification requires. It would be confusing if some platforms gave more precision than others. When you use the strictfp modifier on a method or class, the compiler generates code that adheres strictly to the Java spec for identical results on all platforms. Without strictfp, is it is slightly laxer, but not so lax as to use the guard bits in the Pentium to give 80 bits of precision. And finally the actual Java Language Specification, §15.4 FP-strict Expressions: Within an FP-strict expression, all intermediate values must be elements of the float value set or the double value set, implying that the results of all FP-strict expressions must be those predicted by IEEE 754 arithmetic on operands represented using single and double formats. Within an expression that is not FP-strict, some leeway is granted for an implementation to use an extended exponent range to represent intermediate results; the net effect, roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow.

不过我个人从来没用过它。

strictfp是一个关键字，可以用作类或方法的非非访问修饰符(但不能用作变量)。将类标记为strictfp意味着类中的任何方法代码都将符合IEEE 754浮点标准规则。

如果没有这个修饰符，方法中使用的浮点可能会以依赖于平台的方式运行。有了它，无论JVM运行在哪个底层平台上，您都可以预测浮点数的行为。缺点是，如果底层平台能够支持更高的精度，那么strictfp方法将无法利用它。

如果您没有将一个类声明为strictfp，那么通过将一个方法声明为strictfp，您仍然可以在逐个方法的基础上获得strictfp行为。

~ SCJP Sun®Java™6认证程序员- Kathy Sierra & Bert Bates ~

'strictfp'关键字用于强制Java中的浮点计算(float或double)的精度显式符合IEEE的754标准。如果不使用strictfp关键字，浮点精度取决于目标平台的硬件。

如果接口或类使用strictfp声明，那么该接口或类中的所有方法和嵌套类型都是隐式strictfp。

参考链接

Strictfp确保在每个平台上从浮点计算中得到完全相同的结果。如果不使用strictfp, JVM实现可以在可用的地方使用额外的精度。

来自JLS:

Within an FP-strict expression, all intermediate values must be elements of the float value set or the double value set, implying that the results of all FP-strict expressions must be those predicted by IEEE 754 arithmetic on operands represented using single and double formats. Within an expression that is not FP-strict, some leeway is granted for an implementation to use an extended exponent range to represent intermediate results; the net effect, roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow.

换句话说，就是要确保Write-Once-Run-Anywhere实际上意味着write - once - get - equal - wrong - results - everywhere。

使用strictfp，您的结果是可移植的，没有它，它们更可能是准确的。

实际上，维基百科上有一篇关于strictfp的好文章，其中有一个指向Java规范中浮点类型、格式和值部分的链接。

字里行间的意思是，如果您没有指定strictfp，那么JVM和JIT编译器就有权按它们想要的方式计算您的浮点计算。为了提高速度，它们很可能将计算委托给您的处理器。在使用strictfp时，计算必须符合IEEE 754算术标准，这在实践中可能意味着JVM将执行计算。

那么为什么要使用strictfp呢?我能看到的一个场景是在分布式应用程序(或多人游戏)中，无论底层硬件或CPU是什么，所有浮点计算都需要是确定的。代价是什么?很可能是执行时间。

strictfp is a modifier which restricts floating point calculations as per IEEE 754. This can be used on whole class like "public strictfp class StrictFpModifierExample{}" or on method "public strictfp void example()".If it is used on class than all methods will follow IEEE 754 and if used on method then particular method will follow IEEE 754. Why it is used??::: As different platforms have different floating point hardware which calculates with more precision and greater range of values than the java specification requires which may produce diffrent output on diffrent plateforms.so it confirms the same output irrespective of diffrent plateforms strictfp also ensures to take advantage of the speed and precision of the extended precision floating-point operations. There is no disadvantage with this keyword we can use when we are doing floating point calculations My last point is --What is IEEE754 in short IEEE 754 defines standard method for both floating point calculations and storage of floating point values in either single (32-bit, used in Java floats) or double (64-bit, used in Java doubles) precision.It also defines norms for intermediate calculations and for extended precision formats.