Proposal: java.lang.runtime.Carrier

[Jim Laskey]

We propose to provide a runtime anonymous carrier class object generator; java.lang.runtime.Carrier. This generator class is designed to share anonymous classes when shapes are similar. For example, if several clients require objects containing two integer fields, then Carrier will ensure that each client generates carrier objects using the same underlying anonymous class.

Providing this mechanism decouples the strategy for carrier class generation from the client facility. One could implement one class per shape; one class for all shapes (with an Object[]), or something in the middle; having this decision behind a bootstrap means that it can be evolved at runtime, and optimized differently for different situations.

Motivation

The String Templates JEP draft<https://bugs.openjdk.java.net/browse/JDK-8273943> proposes the introduction of a TemplatedString object for the primary purpose of carrying the template and associated values derived from a template literal. To avoid value boxing, early prototypes described these carrierobjects using per-callsite anonymous classes shaped by value types, The use of distinct anonymous classes here is overkill, especially considering that many of these classes are similar; containing one or two object fields and/or one or two integral fields. Pattern matching has a similar issue when carrying the values for the holes of a pattern. With potentially hundreds (thousands?) of template literals or patterns per application, we need to find an alternate approach for these value carriers.

Description

In general terms, the Carrier class simply caches anonymous classes keyed on shape. To further increase similarity in shape, the ordering of value types is handled by the API and not in the underlying anonymous class. If one client requires an object with one object value and one integer value and a second client requires an object with one integer value and one object value, then both clients will use the same underlying anonymous class. Further, types are folded as either integer (byte, short, int, boolean, char, float), long (long, double) or object. [We've seen that performance hit by folding the long group into the integer group is significant, hence the separate group.]

The Carrier API uses MethodType parameter types to describe the shape of a carrier. This incorporates with the primary use case where bootstrap methods need to capture indy non-static arguments. The API has three static methods;

// Return a constructor MethodHandle for a carrier with components
// aligning with the parameter types of the supplied methodType.
static MethodHandle constructor(MethodType methodType)

// Return a component getter MethodHandle for component i.
static MethodHandle component(MethodType methodType, int i)

// Return component getter MethodHandles for all the carrier's components.
static MethodHandle[] components(MethodType methodType)

Examples

import java.lang.runtime.Carrier;
...

// Define the carrier description.
MethodType methodType =
    MethodType.methodType(Object.class, byte.class, short.class,
            char.class, int.class, long.class,
            float.class, double.class,
            boolean.class, String.class);

// Fetch the carrier constructor.
MethodHandle constructor = Carrier.constructor(methodType);

// Create a carrier object.
Object object = (Object)constructor.invokeExact((byte)0xFF, (short)0xFFFF,
        'C', 0xFFFFFFFF, 0xFFFFFFFFFFFFFFFFL,
        1.0f / 3.0f, 1.0 / 3.0,
        true, "abcde");

// Get an array of accessors for the carrier object.
MethodHandle[] components = Carrier.components(methodType);

// Access fields.
byte b = (byte)components[0].invokeExact(object);
short s = (short)components[1].invokeExact(object);
char c =(char)components[2].invokeExact(object);
int i = (int)components[3].invokeExact(object);
long l = (long)components[4].invokeExact(object);
float f =(float)components[5].invokeExact(object);
double d = (double)components[6].invokeExact(object);
boolean tf (boolean)components[7].invokeExact(object);
String s = (String)components[8].invokeExact(object));

// Access a specific field.
MethodHandle component = Carrier.component(methodType, 3);
int ii = (int)component.invokeExact(object);

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