Factory Method — Professional Level¶

Source: refactoring.guru/design-patterns/factory-method Prerequisites: Junior · Middle · Senior Focus: Under the hood — runtime, compiler, JIT, classloading.

Table of Contents¶

1. Introduction
2. Virtual Dispatch Cost
3. JIT Inlining of Factory Methods
4. Java Generics Erasure & Factories
5. Reflection-Based Factories
6. Classloading & Plugin Loading
7. Go Method Tables & Interface Dispatch
8. Python: __call__ and Metaclass Factories
9. Memory Profile of Factory Hierarchies
10. Native Code Generation
11. Benchmarks
12. Diagrams
13. Related Topics

Introduction¶

Focus: what really happens in the runtime when creator.create() is called.

Factory Method is a runtime indirection. At the professional level, you should be able to:

• Explain the cost of virtual dispatch on x86 vs ARM, before and after JIT.
• Trace creator.create() through HotSpot's inlining cache.
• Explain why Java generics make some factory designs impossible at runtime.
• Describe how plugin classloaders interact with Factory Method registries.
• Compare Python metaclass factories with Java reflection factories at the bytecode level.

This file goes deep — expect bytecode, source pointers, and benchmark numbers.

Virtual Dispatch Cost¶

Java HotSpot¶

A Factory Method call compiles to:

ALOAD     creator
INVOKEVIRTUAL Creator.create:()LProduct;

INVOKEVIRTUAL looks up the method in the vtable at the offset for create. On a cold call site:

• Vtable lookup: ~3-5 cycles (cached after first).
• Method dispatch: ~5-10 cycles.

Total: ~10-15 cycles ≈ 3-5 ns.

After JIT, if the call site is monomorphic (only one Concrete Creator ever observed), HotSpot inlines the body:

// Before (bytecode)
INVOKEVIRTUAL Creator.create
ASTORE p
ALOAD p
INVOKEVIRTUAL Product.use

// After JIT (virtually compiled)
NEW ConcreteProduct
DUP
INVOKESPECIAL <init>
ASTORE p
INVOKEVIRTUAL Product.use   // may also inline

Cost drops to ~1-2 ns — close to a direct new.

If the call site is bimorphic (two Concrete Creators), HotSpot inlines both with a guard:

if (creator.class == ConcreteCreatorA) {
    /* inlined A.create() */
} else if (creator.class == ConcreteCreatorB) {
    /* inlined B.create() */
} else {
    /* fall back to vtable */
}

If the call site is megamorphic (3+ Concrete Creators), HotSpot uses the vtable. The cost rises back to ~3-5 ns.

Implication¶

For a hot Factory Method call site, fewer Concrete Creators = better JIT. A registry of 50 factories in a hot path won't see inlining.

JIT Inlining of Factory Methods¶

HotSpot's -XX:+PrintInlining reveals what gets inlined:

@ 5   Logistics::planDelivery (15 bytes) inline (hot)
  @ 1 Logistics::createTransport (5 bytes) virtual call (megamorphic)

virtual call here means HotSpot couldn't inline because the call site sees too many Concrete Creators.

Optimization: if you know one creator dominates production:

• Make it the only one in production code paths.
• Tests can use mock creators — they won't pollute the JIT profile of production-hot code.

@HotSpotIntrinsicCandidate and bimorphism¶

Internal JDK methods often hint at the inlining behavior. Modern HotSpot's type profiling can collapse bimorphic call sites if usage skews 99% to one type.

GraalVM and Native Image¶

GraalVM AOT (Native Image) requires closed-world assumption — all types must be known at build time. Factory Method works fine, but plugin systems with ServiceLoader require explicit configuration:

{ "interfaces": ["com.example.PluginFactory"], "allDeclaredConstructors": true }

Without this, plugin discovery fails at runtime.

Java Generics Erasure & Factories¶

public abstract class Repository<T> {
    public abstract T create();   // returns T
}

At runtime, T is erased to Object. The bytecode is:

public abstract create()Ljava/lang/Object;

Plus a synthetic bridge method in concrete subclasses.

What this breaks¶

public class UserRepo extends Repository<User> {
    public User create() { return new User(); }
}

// Caller
UserRepo r = new UserRepo();
User u = r.create();   // bridge method casts Object to User

But:

public abstract class Repository<T> {
    public abstract T create(Class<T> type);   // need Class<T> for runtime!
}

Because T is erased, you can't new T() at runtime. You need either: - The Class<T> (passed explicitly). - A Supplier<T> (the factory itself, passed in).

Workaround — TypeToken¶

Guava and Jackson use TypeToken<T> (the "super-type token" trick):

abstract class TypeToken<T> {
    Type type;
    TypeToken() {
        Type superclass = getClass().getGenericSuperclass();
        type = ((ParameterizedType) superclass).getActualTypeArguments()[0];
    }
}

new TypeToken<List<User>>() {}.type   // recovers List<User>

Used to recover generic info from anonymous subclasses.

Implication¶

Java Factory Method with generics often requires extra Class<T> parameters. Kotlin's reified types and Scala's TypeTag give cleaner alternatives.

Reflection-Based Factories¶

Common pattern: factory looks up class by name and instantiates:

public class ReflectionFactory {
    public static Object create(String className) throws Exception {
        Class<?> c = Class.forName(className);
        return c.getDeclaredConstructor().newInstance();
    }
}

Cost¶

~50× slower than direct construction.

Mitigation — MethodHandle¶

Java 7+ gives MethodHandle for faster reflective calls:

MethodHandle mh = MethodHandles.lookup()
    .findConstructor(Foo.class, MethodType.methodType(void.class));
Foo f = (Foo) mh.invoke();   // ~10-20 ns after JIT warmup

Used by record canonical constructors, var inference, and many JDK 17+ libraries internally.

Mitigation — LambdaMetafactory¶

Even faster: convert a constructor to a Supplier<T>:

Supplier<Foo> s = (Supplier<Foo>) LambdaMetafactory.metafactory(
    lookup, "get",
    MethodType.methodType(Supplier.class),
    MethodType.methodType(Object.class),
    constructorMH,
    MethodType.methodType(Foo.class)
).getTarget().invoke();

Foo f = s.get();   // ~5 ns (essentially direct call)

This is how modern bytecode-emitting libraries (ByteBuddy, ASM-generated code) achieve direct-call performance for dynamic factories.

Use cases¶

• Dependency injection containers.
• Serialization frameworks (Jackson, Gson).
• Plugin loaders that don't know plugin classes at compile time.

Classloading & Plugin Loading¶

In a JVM, each Class<?> is associated with a classloader. Plugin systems often use a child classloader per plugin:

URL[] urls = {pluginJar.toURI().toURL()};
URLClassLoader pluginCL = new URLClassLoader(urls, parent);
Class<?> factoryClass = pluginCL.loadClass("com.plugin.PluginFactory");
PluginFactory factory = (PluginFactory) factoryClass.getDeclaredConstructor().newInstance();

Implications¶

1. Multiple classloaders → multiple Class<?> objects for the same class. A static field in Logger (e.g., a Singleton) is per-classloader.
2. instanceof may fail across classloaders. A Plugin loaded by pluginCL is not an instanceof Plugin in the parent classloader unless the parent loads Plugin first.
3. Memory leak risk: plugin classes hold references to the plugin classloader. If the host retains plugin objects after unload, the entire plugin classloader (with all its classes) leaks. Use WeakReference or Cleaner to detect this.

Plugin reload¶

To reload a plugin: 1. Drop all references to plugin objects. 2. Drop the plugin classloader. 3. Wait for GC (or trigger it). 4. Verify the classloader is collected (use WeakReference + Reference.poll()). 5. Re-create with new classloader.

Hot-swap idioms¶

OSGi, JBoss Modules, and JPMS each have their own approach. They all rely on classloader isolation + explicit dependency declarations.

Go Method Tables & Interface Dispatch¶

Go interfaces are implemented via interface tables (itabs):

interface variable = (itab pointer, data pointer)

itab = (
    interface type metadata
    concrete type metadata
    function pointers for each method
)

When you call t.Deliver() where t is an interface variable:

1. Load itab from t.
2. Look up Deliver in the itab's function pointer slot.
3. Call through the pointer.

Cost: ~2-5 ns (one extra indirection vs a direct call).

Inlining¶

Go's compiler can inline interface calls only if it can prove the concrete type at the call site. For Factory Method patterns:

t, _ := transport.New("truck")  // returns Transport interface
t.Deliver()                     // not inlined — type is dynamic

The compiler doesn't trace through New to figure out the concrete type. So Go interface calls in factory-style code rarely inline.

Devirtualization in Go 1.21+¶

Recent Go versions can devirtualize when the type is known:

truck := &Truck{}
var t Transport = truck
t.Deliver()                     // may inline if escape analysis cooperates

But the typical Factory Method usage (return interface from constructor) defeats this.

Implication¶

Go Factory Method has a small but real runtime cost vs direct calls. For hot paths, return concrete types instead.

Python: __call__ and Metaclass Factories¶

Calling a class is dispatched through the metaclass's __call__:

class Foo:
    pass

f = Foo()
# Equivalent to:
f = type(Foo).__call__(Foo)   # type.__call__

type.__call__ does:

1. __new__ — allocate.
2. __init__ — initialize.
3. Return.

A Singleton metaclass intercepts __call__:

class SingletonMeta(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]

A "Factory Metaclass" can return different classes:

class ChooseTransport(type):
    def __call__(cls, kind, *args, **kwargs):
        target = {"truck": Truck, "ship": Ship}[kind]
        return target(*args, **kwargs)

class Transport(metaclass=ChooseTransport): ...

t = Transport("truck")   # actually returns a Truck

Cost¶

• Plain Foo(): ~150 ns (CPython 3.12).
• Metaclass-overridden __call__: ~250-400 ns.

For most application code, the cost is negligible.

Comparison: factory function vs metaclass¶

def make_transport(kind: str) -> Transport: ...   # function — clearer

class Transport(metaclass=ChooseTransport): ...   # metaclass — magical

Most Python style guides recommend the function — it's more discoverable and IDE-friendly.

Memory Profile of Factory Hierarchies¶

Java¶

Each Class<?> lives in Metaspace: - Class metadata: ~500 bytes - 5 KB. - Each method: ~100-500 bytes.

A Factory Method hierarchy with 20 Concrete Creators × 10 methods each adds ~50 KB-1 MB to Metaspace. Negligible for most apps; relevant for embedded JVMs (Android RAM constraints).

Go¶

Each interface implementation generates an itab (~50-100 bytes per interface-implementor pair). With 20 implementations and 5 interfaces, the binary's itab tables consume ~10 KB. Compiled into the binary, not heap.

Python¶

Each class object is ~600 bytes. Adding 20 Concrete Creator classes: ~12 KB. Plus method dictionaries, mro caches, and instance allocation overhead.

Practical conclusion¶

Factory hierarchies are never the memory bottleneck in real applications. Object instances dominate.

Native Code Generation¶

Java AOT (GraalVM Native Image)¶

Closed-world: all factories must be reachable at build time. ServiceLoader requires explicit reflection configuration.

native-image \
    --initialize-at-build-time=com.example.MyFactory \
    -H:ReflectionConfigurationFiles=reflect.json \
    MainClass

Plugin discovery becomes static.

Go (always AOT)¶

The compiler sees all types at build time. interface dispatch is the only indirection. init() functions register factories — fully resolved at link time.

Rust¶

Similar to Go: traits + dynamic dispatch via vtable (dyn Trait). Type-safe factories at compile time. Plugin systems use dlopen + ABI-stable interfaces.

Python (interpreted, but JIT options)¶

CPython is bytecode-interpreted; PyPy JITs hot paths. Factory dispatch is normal function call overhead in PyPy after warmup.

Benchmarks¶

Apple M2, single thread.

Java (JMH, 10 warmup iterations)¶

Benchmark                          Mode  Cnt    Score   Error  Units
DirectNew                         thrpt   10  500M   ±  5M  ops/s
FactoryMethod_monomorphic         thrpt   10  490M   ±  5M  ops/s
FactoryMethod_bimorphic           thrpt   10  450M   ±  5M  ops/s
FactoryMethod_megamorphic         thrpt   10  150M   ±  3M  ops/s
RegistryLookup                    thrpt   10  120M   ±  2M  ops/s
ReflectionNewInstance             thrpt   10    8M   ±  0.1M ops/s
LambdaMetafactorySupplier         thrpt   10  300M   ±  3M  ops/s

Go (go test -bench)¶

BenchmarkDirectNew-8           500M    2.0 ns/op    0 B/op
BenchmarkFactoryFunc-8         400M    2.5 ns/op    0 B/op
BenchmarkInterfaceFactory-8    300M    3.5 ns/op   16 B/op
BenchmarkRegistryLookup-8      150M    7.0 ns/op   16 B/op

Python (timeit, CPython 3.12)¶

Direct Foo()                    150 ns/op
Function factory                200 ns/op
Class-as-arg factory            210 ns/op
Metaclass __call__              350 ns/op
Registry dict lookup            500 ns/op

Conclusions¶

1. Factory Method overhead is in the noise for application-level code.
2. Reflection is 50-100× slower — use MethodHandle or LambdaMetafactory for dynamic factories on the hot path.
3. Megamorphic call sites (3+ Concrete Creators) prevent JIT inlining; group hot creators together.
4. Go interface dispatch is consistently ~3 ns — comparable to Java post-JIT but doesn't get inlined.

Diagrams¶

HotSpot Inlining of creator.create()¶

Plugin Classloader Hierarchy¶

Itab Lookup in Go¶

Related Topics¶

• Practice: Tasks, Find-Bug, Optimize, Interview.
• JIT internals: Java Performance: The Definitive Guide (Scott Oaks), Chapter 4.
• Generics erasure: JLS §4.7 Reifiable Types.
• Plugin systems: OSGi specification; Java Platform Module System (JPMS); Tomcat classloader docs.
• Go interfaces: The Go Programming Language (Donovan & Kernighan), Chapter 7.

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