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/**
* Defines the API for dynamic linking of high-level operations on objects.
* <p>
* Dynalink is a library for dynamic linking of high-level operations on objects.
* These operations include "read a property",
* "write a property", "invoke a function" and so on. Dynalink is primarily
* useful for implementing programming languages where at least some expressions
* have dynamic types (that is, types that can not be decided statically), and
* the operations on dynamic types are expressed as
* {@linkplain java.lang.invoke.CallSite call sites}. These call sites will be
* linked to appropriate target {@linkplain java.lang.invoke.MethodHandle method handles}
* at run time based on actual types of the values the expressions evaluated to.
* These can change between invocations, necessitating relinking the call site
* multiple times to accommodate new types; Dynalink handles all that and more.
* <p>
* Dynalink supports implementation of programming languages with object models
* that differ (even radically) from the JVM's class-based model and have their
* custom type conversions.
* <p>
* Dynalink is closely related to, and relies on, the {@link java.lang.invoke}
* package.
* <p>
*
* While {@link java.lang.invoke} provides a low level API for dynamic linking
* of {@code invokedynamic} call sites, it does not provide a way to express
* higher level operations on objects, nor methods that implement them. These
* operations are the usual ones in object-oriented environments: property
* access, access of elements of collections, invocation of methods and
* constructors (potentially with multiple dispatch, e.g. link- and run-time
* equivalents of Java overloaded method resolution). These are all functions
* that are normally desired in a language on the JVM. If a language is
* statically typed and its type system matches that of the JVM, it can
* accomplish this with use of the usual invocation, field access, etc.
* instructions (e.g. {@code invokevirtual}, {@code getfield}). However, if the
* language is dynamic (hence, types of some expressions are not known until
* evaluated at run time), or its object model or type system don't match
* closely that of the JVM, then it should use {@code invokedynamic} call sites
* instead and let Dynalink manage them.
* <h2>Example</h2>
* Dynalink is probably best explained by an example showing its use. Let's
* suppose you have a program in a language where you don't have to declare the
* type of an object and you want to access a property on it:
* <pre>
* var color = obj.color;
* </pre>
* If you generated a Java class to represent the above one-line program, its
* bytecode would look something like this:
* <pre>
* aload 2 // load "obj" on stack
* invokedynamic "GET:PROPERTY:color"(Object)Object // invoke property getter on object of unknown type
* astore 3 // store the return value into local variable "color"
* </pre>
* In order to link the {@code invokedynamic} instruction, we need a bootstrap
* method. A minimalist bootstrap method with Dynalink could look like this:
* <pre>
* import java.lang.invoke.*;
* import jdk.dynalink.*;
* import jdk.dynalink.support.*;
*
* class MyLanguageRuntime {
* private static final DynamicLinker dynamicLinker = new DynamicLinkerFactory().createLinker();
*
* public static CallSite bootstrap(MethodHandles.Lookup lookup, String name, MethodType type) {
* return dynamicLinker.link(
* new SimpleRelinkableCallSite(
* new CallSiteDescriptor(lookup, parseOperation(name), type)));
* }
*
* private static Operation parseOperation(String name) {
* ...
* }
* }
* </pre>
* There are several objects of significance in the above code snippet:
* <ul>
* <li>{@link jdk.dynalink.DynamicLinker} is the main object in Dynalink, it
* coordinates the linking of call sites to method handles that implement the
* operations named in them. It is configured and created using a
* {@link jdk.dynalink.DynamicLinkerFactory}.</li>
* <li>When the bootstrap method is invoked, it needs to create a
* {@link java.lang.invoke.CallSite} object. In Dynalink, these call sites need
* to additionally implement the {@link jdk.dynalink.RelinkableCallSite}
* interface. "Relinkable" here alludes to the fact that if the call site
* encounters objects of different types at run time, its target will be changed
* to a method handle that can perform the operation on the newly encountered
* type. {@link jdk.dynalink.support.SimpleRelinkableCallSite} and
* {@link jdk.dynalink.support.ChainedCallSite} (not used in the above example)
* are two implementations already provided by the library.</li>
* <li>Dynalink uses {@link jdk.dynalink.CallSiteDescriptor} objects to
* preserve the parameters to the bootstrap method: the lookup and the method type,
* as it will need them whenever it needs to relink a call site.</li>
* <li>Dynalink uses {@link jdk.dynalink.Operation} objects to express
* dynamic operations. It does not prescribe how would you encode the operations
* in your call site, though. That is why in the above example the
* {@code parseOperation} function is left empty, and you would be expected to
* provide the code to parse the string {@code "GET:PROPERTY:color"}
* in the call site's name into a named property getter operation object as
* {@code StandardOperation.GET.withNamespace(StandardNamespace.PROPERTY).named("color")}.
* </ul>
* <p>What can you already do with the above setup? {@code DynamicLinkerFactory}
* by default creates a {@code DynamicLinker} that can link Java objects with the
* usual Java semantics. If you have these three simple classes:
* <pre>
* public class A {
* public String color;
* public A(String color) { this.color = color; }
* }
*
* public class B {
* private String color;
* public B(String color) { this.color = color; }
* public String getColor() { return color; }
* }
*
* public class C {
* private int color;
* public C(int color) { this.color = color; }
* public int getColor() { return color; }
* }
* </pre>
* and you somehow create their instances and pass them to your call site in your
* programming language:
* <pre>
* for each(var obj in [new A("red"), new B("green"), new C(0x0000ff)]) {
* print(obj.color);
* }
* </pre>
* then on first invocation, Dynalink will link the {@code .color} getter
* operation to a field getter for {@code A.color}, on second invocation it will
* relink it to {@code B.getColor()} returning a {@code String}, and finally on
* third invocation it will relink it to {@code C.getColor()} returning an {@code int}.
* The {@code SimpleRelinkableCallSite} we used above only remembers the linkage
* for the last encountered type (it implements what is known as a <i>monomorphic
* inline cache</i>). Another already provided implementation,
* {@link jdk.dynalink.support.ChainedCallSite} will remember linkages for
* several different types (it is a <i>polymorphic inline cache</i>) and is
* probably a better choice in serious applications.
* <h2>Dynalink and bytecode creation</h2>
* {@code CallSite} objects are usually created as part of bootstrapping
* {@code invokedynamic} instructions in bytecode. Hence, Dynalink is typically
* used as part of language runtimes that compile programs into Java
* {@code .class} bytecode format. Dynalink does not address the aspects of
* either creating bytecode classes or loading them into the JVM. That said,
* Dynalink can also be used without bytecode compilation (e.g. in language
* interpreters) by creating {@code CallSite} objects explicitly and associating
* them with representations of dynamic operations in the interpreted program
* (e.g. a typical representation would be some node objects in a syntax tree).
* <h2>Available operations</h2>
* Dynalink defines several standard operations in its
* {@link jdk.dynalink.StandardOperation} class. The linker for Java
* objects can link all of these operations, and you are encouraged to at
* minimum support and use these operations in your language too. The
* standard operations {@code GET} and {@code SET} need to be combined with
* at least one {@link jdk.dynalink.Namespace} to be useful, e.g. to express a
* property getter, you'd use {@code StandardOperation.GET.withNamespace(StandardNamespace.PROPERTY)}.
* Dynalink defines three standard namespaces in the {@link jdk.dynalink.StandardNamespace} class.
* To associate a fixed name with an operation, you can use
* {@link jdk.dynalink.NamedOperation} as in the previous example:
* {@code StandardOperation.GET.withNamespace(StandardNamespace.PROPERTY).named("color")}
* expresses a getter for the property named "color".
* <h2>Operations on multiple namespaces</h2>
* Some languages might not have separate namespaces on objects for
* properties, elements, and methods, and a source language construct might
* address several of them at once. Dynalink supports specifying multiple
* {@link jdk.dynalink.Namespace} objects with {@link jdk.dynalink.NamespaceOperation}.
* <h2>Language-specific linkers</h2>
* Languages that define their own object model different than the JVM
* class-based model and/or use their own type conversions will need to create
* their own language-specific linkers. See the {@link jdk.dynalink.linker}
* package and specifically the {@link jdk.dynalink.linker.GuardingDynamicLinker}
* interface to get started.
* <h2>Dynalink and Java objects</h2>
* The {@code DynamicLinker} objects created by {@code DynamicLinkerFactory} by
* default contain an internal instance of
* {@code BeansLinker}, which is a language-specific linker
* that implements the usual Java semantics for all of the above operations and
* can link any Java object that no other language-specific linker has managed
* to link. This way, all language runtimes have built-in interoperability with
* ordinary Java objects. See {@link jdk.dynalink.beans.BeansLinker} for details
* on how it links the various operations.
* <h2>Cross-language interoperability</h2>
* A {@code DynamicLinkerFactory} can be configured with a
* {@linkplain jdk.dynalink.DynamicLinkerFactory#setClassLoader(ClassLoader) class
* loader}. It will try to instantiate all
* {@link jdk.dynalink.linker.GuardingDynamicLinkerExporter} classes visible to
* that class loader and compose the linkers they provide into the
* {@code DynamicLinker} it creates. This allows for interoperability between
* languages: if you have two language runtimes A and B deployed in your JVM and
* they export their linkers through the above mechanism, language runtime A
* will have a language-specific linker instance from B and vice versa inside
* their {@code DynamicLinker} objects. This means that if an object from
* language runtime B gets passed to code from language runtime A, the linker
* from B will get a chance to link the call site in A when it encounters the
* object from B.
*
* @uses jdk.dynalink.linker.GuardingDynamicLinkerExporter
*
* @moduleGraph
* @since 9
*/
module jdk.dynalink {
requires java.logging;
exports jdk.dynalink;
exports jdk.dynalink.beans;
exports jdk.dynalink.linker;
exports jdk.dynalink.linker.support;
exports jdk.dynalink.support;
uses jdk.dynalink.linker.GuardingDynamicLinkerExporter;
}