Subsequent Modi cation of Final Fields in Java

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1753 Subsequent Modi cation of Final Fields
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In some cases, such as deserialization, the system will need to change the nal elds of an object after construction Final elds can be changed via re ection and other implementation dependent means The only pattern in which this has reasonable semantics is one in which an object is constructed and then the nal elds of the object are updated The object should not be made visible to other threads, nor should the nal elds be read, until all updates to the nal elds of the object are complete Freezes of a nal eld occur both at the end of the constructor in which the nal eld is set, and immediately after each modi cation of a nal eld via re ection or other special mechanism Even then, there are a number of complications If a nal eld is initialized to a compile-time constant in the eld declaration, changes to the nal eld may not be observed, since uses of that nal eld are replaced at compile time with the compile-time constant Another problem is that the speci cation allows aggressive optimization of nal elds Within a thread, it is permissible to reorder reads of a nal eld with those modi cations of a nal eld that do not take place in the constructor
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Subsequent Modi cation of Final Fields
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For example, consider the following code fragment:
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class A { final int x; A() { x = 1; } int f() { return d(this,this); } int d(A a1, A a2) { int i = a1x; g(a1); int j = a2x; return j - i; } static void g(A a) { // uses reflection to change ax to 2 } }
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An implementation may provide a way to execute a block of code in a nal eld safe context If an object is constructed within a nal eld safe context, the reads of a nal eld of that object will not be reordered with modi cations of that nal eld that occur within that nal eld safe context A nal eld safe context has additional protections If a thread has seen an incorrectly published reference to an object that allows the thread to see the default value of a nal eld, and then, within a nal- eld safe context, reads a properly published reference to the object, it will be guaranteed to see the correct value of the nal eld In the formalism, code executed within a nal- eld safe context is treated as a separate thread (for the purposes of nal eld semantics only) In an implementation, a compiler should not move an access to a nal eld into or out of a nal- eld safe context (although it can be moved around the execution of such a context, so long as the object is not constructed within that context) One place where use of a nal- eld safe context would be appropriate is in an executor or thread pool By executing each Runnable in a separate nal eld safe context, the executor could guarantee that incorrect access by one Runnable 577
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In the d() method, the compiler is allowed to reorder the reads of x and the call to g() freely Thus, A()f() could return -1, 0 or 1
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to a object o won't remove nal eld guarantees for other Runnables handled by the same executor 1754 Write Protected Fields Normally, nal static elds may not be modi ed However Systemin, Systemout, and Systemerr are nal static elds that, for legacy reasons, must be allowed to be changed by the methods SystemsetIn, SystemsetOut and SystemsetErr We refer to these elds as being write-protected to distinguish them from ordinary nal elds The compiler needs to treat these elds differently from other nal elds For example, a read of an ordinary nal eld is ``immune'' to synchronization: the barrier involved in a lock or volatile read does not have to affect what value is read from a nal eld Since the value of write-protected elds may be seen to change, synchronization events should have an effect on them Therefore, the semantics dictate that these elds be treated as normal elds that cannot be changed by user code, unless that user code is in the System class
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