Sundararajan's Weblog

  • Java
    November 10, 2015

Dynamic linker API for the Java platform (JEP 276)

JEP 276 defines dynamic linker API for Java. This JEP provides "facility for linking high-level operations on objects such as "read a property", "write a property", "invoke a callable object", etc., expressed as names in INVOKEDYNAMIC call sites. Provide a default linker for the usual semantics of these operations on plain Java objects, as well as a facility for installing language-specific linkers.

Nashorn JavaScript engine already uses "dynalink" library for linking properties, indexed access, calls on it's (script) objects as well as "foreign"/"host" Java objects (POJOs). With JEP-276, dynalink is exposed as a public (JDK specific) API in the java9 module named "jdk.dynalink".

Right now, the source code for JEP-276 lives in "jdk9 sandbox" OpenJDK repository (http://hg.openjdk.java.net/jdk9/sandbox) in the branched named "JEP-276-branch". This will eventually go into jdk9 repository. If you want to play with "dynalink" API, you can check out this forest and build "JEP-276-branch"

To play with any Java API, these days I use "jshell" (jshell) tool. The following is a jshell repl sample to demonstrate dynalink API from Java code.

import java.lang.invoke.*
// dynalink API lives in these packages
import jdk.dynalink.*
import jdk.dynalink.support.*
// dynamic 'operation' for a callsite. 'length' property
Operation op = new NamedOperation(StandardOperation.GET_PROPERTY, "length")
// method type of operation to be linked - length is 'int' value
MethodType mt = MethodType.methodType(int.class, Object.class)
// callsite descriptor
CallSiteDescriptor desc = new CallSiteDescriptor(MethodHandles.publicLookup(),
op, mt)
// callsite
SimpleRelinkableCallSite cs = new SimpleRelinkableCallSite(desc)
// create a linker factory
DynamicLinkerFactory fac = new DynamicLinkerFactory()
// create dynalink linker
DynamicLinker linker = fac.createLinker()
// link the callsite
// invoke it!
printf("array size %d\n", (int)cs.getTarget().invoke(new String[10]))
import java.util.ArrayList
// make a list and populate two elements
ArrayList<String> al = new ArrayList<>()
// get 'length' of array list - which is nothing but size
printf("list size %d\n", (int)cs.getTarget().invoke(al))

The above repl prints "array size 10" and "list size 2" respectively. Note that "length" property is relinked automatically to be array length and then ArrayList size (when a different "this" object is passed). This linking (and relinking) of Java objects is handled by the "java beans linker" that comes with the dynalink implementation.

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