org.perfectable.introspection.proxy.LazyInitialization Maven / Gradle / Ivy
package org.perfectable.introspection.proxy;
import java.lang.reflect.Method;
import java.util.Optional;
import javax.annotation.Nullable;
import com.google.errorprone.annotations.concurrent.LazyInit;
import static org.perfectable.introspection.Introspections.introspect;
/**
* Helper class to create proxy which delegate invocation to lazily-initialized instance.
*
* Proxies are created by {@link #createProxy}.
*/
public final class LazyInitialization {
/**
* Additional interface that all proxies created by {@link #createProxy} will implement.
*
* @param type of actual delegation target.
*/
public interface Proxy {
/**
* Extracts delegation target.
*
* @return target if it was initialized, empty if proxy hasn't yet been initialized.
*/
Optional extractInstance();
}
/**
* Provider interface for proxy delegation target.
*
* @param type of target
*/
@FunctionalInterface
public interface Initializer {
/**
* Initializes the actual target and returns it.
*
* In ideal scenario this method is called once for each proxy.
*
* @return Delegation target for created proxy
*/
T initialize();
}
/**
* Creates a proxy that forwards invocations to lazily initialized instance.
*
*
On any method call on resulting proxy it will either initialize instance and cache it, or use existing
* instance, and delegate the call to it, returning the result or rethrowing exception of the call. There are
* two exceptions of this rule:
*
* - {@link Object#finalize} is always ignored when called on proxy
* - Methods from {@link Proxy} interface are treated specially and handled with separate route. They
* won't necessarily initialize the proxy and won't be forwarded to the target.
*
*
* Proxies will have {@code resultClass} as a superclass/superinterface. Additionally, proxy class will implement
* {@link Proxy} interface. This wont be reflected in the compile-time type, but it can be checked by instanceof
* and it can be casted to it.
*
* @param resultClass class or interface that proxy has to implement. This is also a superclass/superinterface of
* initialized instance
* @param initializer method of initializing actual delegation target
* @param type of proxy
* @return proxy object
*/
public static T createProxy(Class resultClass, Initializer initializer) {
LazyInitializationHandler handler = LazyInitializationHandler.create(initializer);
return ProxyBuilder.forType(resultClass).withInterface(Proxy.class).instantiate(handler);
}
private static final class LazyInitializationHandler implements InvocationHandler> {
private static final Method EXTRACT_INSTANCE_METHOD =
introspect(Proxy.class).methods().named("extractInstance").parameters().unique();
private final Initializer initializer;
@LazyInit
private transient T instance;
public static LazyInitializationHandler create(Initializer initializer) {
return new LazyInitializationHandler<>(initializer);
}
private LazyInitializationHandler(Initializer initializer) {
this.initializer = initializer;
}
@Nullable
@Override
public Object handle(MethodInvocation invocation) throws Throwable {
return invocation.decompose(this::replaceInvocation).invoke();
}
private Invocation replaceInvocation(Method method,
@SuppressWarnings("unused") @Nullable T receiver,
Object... arguments) {
if (EXTRACT_INSTANCE_METHOD.equals(method)) {
return () -> Optional.ofNullable(instance);
}
if (this.instance == null) {
this.instance = this.initializer.initialize();
}
return MethodInvocation.of(method, this.instance, arguments);
}
}
private LazyInitialization() {
// utility class
}
}