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package org.checkerframework.dataflow.qual;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* A method is called <em>side-effect-free</em> if it has no visible
* side-effects, such as setting a field of an object that existed before
* the method was called.
* <p>
* Only the visible side-effects are important. The method is allowed to cache
* the answer to a computationally expensive query, for instance. It is also
* allowed to modify newly-created objects, and a constructor is
* side-effect-free if it does not modify any objects that existed before
* it was called.
* <p>
* This annotation is important to pluggable type-checking because if some
* fact about an object is known before a call to such a method, then the
* fact is still known afterwards, even if the fact is about some non-final
* field. When any non-<tt>@SideEffectFree</tt> method is called, then a
* pluggable type-checker must assume that any field of any accessible
* object might have been modified, which annuls the effect of
* flow-sensitive type refinement and prevents the pluggable type-checker
* from making conclusions that are obvious to a programmer.
* <p>
* Also see {@link Pure}, which means both side-effect-free and {@link
* Deterministic}.
* <p>
* <b>Analysis:</b>
* The Checker Framework performs a conservative analysis to verify a
* <tt>@SideEffectFree</tt> annotation.
* The Checker Framework issues a warning
* if the method uses any of the following Java constructs:
* <ol>
* <li>Assignment to any expression, except for local variables and method
* parameters.
* <li>A method invocation of a method that is not <tt>@SideEffectFree</tt>.
* <li>Construction of a new object where the constructor is not <tt>@SideEffectFree</tt>.
* </ol>
* These rules are conservative: any code that passes the checks is
* side-effect-free, but the Checker Framework may issue false positive
* warnings, for code that uses one of the forbidden constructs but is
* side-effect-free nonetheless. In particular, a method that caches its
* result will be rejected.
* <p>
*
* In fact, the rules are so conservative that checking is currently
* disabled by default, but can be enabled via the
* <tt>-AcheckPurityAnnotations</tt> command-line option.
* <p>
*
* @checker_framework.manual #type-refinement-purity Side effects, determinism, purity, and flow-sensitive analysis
*
* @author Stefan Heule
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.METHOD, ElementType.CONSTRUCTOR })
public @interface SideEffectFree {
}
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