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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; |
| } |
| |
