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SciJava Common is a shared library for SciJava software. It provides a plugin framework, with an extensible mechanism for service discovery, backed by its own annotation processor, so that plugins can be loaded dynamically. It is used by downstream projects in the SciJava ecosystem, such as ImageJ and SCIFIO.
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package org.scijava.service;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import org.scijava.AbstractContextual;
import org.scijava.Context;
import org.scijava.Optional;
import org.scijava.event.EventHandler;
import org.scijava.event.EventService;
import org.scijava.log.LogService;
import org.scijava.log.StderrLogService;
import org.scijava.plugin.Parameter;
import org.scijava.plugin.PluginInfo;
import org.scijava.service.event.ServicesLoadedEvent;
import org.scijava.util.ClassUtils;
/**
* Helper class for discovering and instantiating available services.
*
* @author Curtis Rueden
*/
public class ServiceHelper extends AbstractContextual {
/** For logging. */
private LogService log;
/**
* Classes to scan when searching for dependencies. Data structure is a map
* with keys being relevant classes, and values being associated priorities.
*/
private final Map, Double> classPoolMap;
/** Classes to scan when searching for dependencies, sorted by priority. */
private final List> classPoolList;
/** Classes to instantiate as services. */
private final List> serviceClasses;
/**
* Whether service loading will fail fast when there is an error instantiating
* a required service.
*/
private final boolean strict;
/**
* Creates a new service helper for discovering and instantiating services.
*
* @param context The application context for which services should be
* instantiated.
*/
public ServiceHelper(final Context context) {
this(context, null);
}
/**
* Creates a new service helper for discovering and instantiating services.
*
* @param context The application context to which services should be added.
* @param serviceClasses The service classes to instantiate.
*/
public ServiceHelper(final Context context,
final Collection> serviceClasses)
{
this(context, serviceClasses, true);
}
/**
* Creates a new service helper for discovering and instantiating services.
*
* @param context The application context to which services should be added.
* @param serviceClasses The service classes to instantiate.
* @param strict Whether service loading will fail fast when there is an error
* instantiating a required service.
*/
public ServiceHelper(final Context context,
final Collection> serviceClasses,
final boolean strict)
{
setContext(context);
log = context.getService(LogService.class);
if (log == null) log = new StderrLogService();
classPoolMap = new HashMap<>();
classPoolList = new ArrayList<>();
findServiceClasses(classPoolMap, classPoolList);
if (classPoolList.isEmpty()) {
log.warn("Class pool is empty: forgot to call Thread#setClassLoader?");
}
this.serviceClasses = new ArrayList<>();
if (serviceClasses == null) {
// load all discovered services
this.serviceClasses.addAll(classPoolList);
}
else {
// load only the services that were explicitly specified
this.serviceClasses.addAll(serviceClasses);
}
this.strict = strict;
}
// -- ServiceHelper methods --
/**
* Ensures all candidate service classes are registered in the index, locating
* and instantiating compatible services as needed.
*
* This is a NxM operation, where N is the number of service classes attached
* to this ServiceHelper, and M is the number of discovered plugins. Multiple
* implementations of a given service subclass can be loaded. For example, if
* FooService, a subclass of {@link Service} (whether abstract, concrete, or
* an interface), appears on the service list, all subclasses of FooService in
* the plugin class pool will be loaded.
*
*
* NB: In typical use, a {@link Context} will only return the highest priority
* implementation for a loaded service. To retrieve the lower priority
* service(s), they must be requested directly (or through a superclass not
* shared with the higher priority implementation). Thus, as these lower
* priority services will go unused in many cases, it is critical that service
* loading (initialization) is as lightweight as possible.
*
*
* @throws IllegalArgumentException if one of the requested services is
* required (i.e., not marked {@link Optional}) but cannot be
* filled.
*/
public void loadServices() {
for (final Class serviceClass : serviceClasses) {
// Load all compatible classes
for (Class c : classPoolList) {
if (serviceClass.isAssignableFrom(c)) loadService(c);
}
// Make sure loadService gets called once on the actual provided class
loadService(serviceClass);
if (LogService.class.isAssignableFrom(serviceClass)) {
final LogService logService = context().getService(LogService.class);
if (logService != null) log = logService;
}
}
final EventService eventService = context().getService(EventService.class);
if (eventService != null) {
eventService.publishLater(new ServicesLoadedEvent());
}
}
/**
* Obtains a service compatible with the given class, instantiating it (and
* registering it in the index) if necessary.
*
* @return an existing compatible service if one is already registered; or
* else a newly created instance of the service with highest priority;
* or null if no suitable service can be created
* @throws IllegalArgumentException if no suitable service can be created and
* the class is required (i.e., not marked {@link Optional})
*/
public S loadService(final Class c) {
return loadService(c, !isOptional(c));
}
/**
* Instantiates a service of the given class, registering it in the index.
*
* @return the newly created service, or null if the given class cannot be
* instantiated
*/
public S createExactService(final Class c) {
return createExactService(c, false);
}
// -- Helper methods --
/**
* Obtains a service compatible with the given class, instantiating it (and
* registering it in the index) if necessary.
*
* @return an existing compatible service if one is already registered; or
* else a newly created instance of the service with highest priority;
* or null if no suitable service can be created
* @throws IllegalArgumentException if no suitable service can be created and
* the {@code required} flag is {@code true}
*/
private S loadService(final Class c,
final boolean required)
{
// if a compatible service already exists, return it
final S service = context().getService(c);
if (service != null) return service;
// scan the class pool for a suitable match
for (final Class serviceClass : classPoolList) {
if (c.isAssignableFrom(serviceClass)) {
// found a match; now instantiate it
@SuppressWarnings("unchecked")
final S result = (S) createExactService(serviceClass, required);
if (required && result == null) {
final String error = "No match: " + serviceClass.getName();
if (strict) throw new IllegalArgumentException(error);
log.error(error);
}
return result;
}
}
if (required && c.isInterface()) {
final String error = "No compatible service: " + c.getName();
if (strict) throw new IllegalArgumentException(error);
log.error(error);
return null;
}
return createExactService(c, required);
}
/**
* Instantiates a service of the given class, registering it in the index.
*
* @return the newly created service, or null if the given class cannot be
* instantiated
*
* @throws IllegalArgumentException if there is an error creating the service
* and the {@code required} flag is {@code true}
*/
private S createExactService(final Class c,
final boolean required)
{
final String name = c.getName();
log.debug("Creating service: " + name, null);
try {
long start = 0, end = 0;
boolean debug = log.isDebug();
if (debug) start = System.currentTimeMillis();
final S service = createServiceRecursively(c);
context().getServiceIndex().add(service);
if (debug) {
end = System.currentTimeMillis();
log.debug("Created service '" + name + "' in " + (end - start) + " ms");
}
return service;
}
catch (final Throwable t) {
if (required) {
final String error = "Invalid service: " + name;
if (strict) throw new IllegalArgumentException(error, t);
log.error(error, t);
}
else if (log.isDebug()) {
// when in debug mode, give full stack trace of invalid services
log.debug("Invalid service: " + name, t);
}
else {
// we emit only a short warning for failing optional services
log.warn("Invalid service: " + name);
}
}
return null;
}
/**
* Instantiates a service of the given class, recursively populating its
* service parameters.
*/
private S createServiceRecursively(final Class c)
throws InstantiationException, IllegalAccessException
{
final S service = c.newInstance();
service.setContext(getContext());
// propagate priority if known
final Double priority = classPoolMap.get(c);
if (priority != null) service.setPriority(priority);
// NB: If there are any @EventHandler annotated methods, we treat the
// EventService as a required dependency, _unless_ there is also an
// EventService field annotated with @Parameter(required = false).
boolean eventServiceRequired = true;
// populate service parameters
final List fields =
ClassUtils.getAnnotatedFields(c, Parameter.class);
for (final Field f : fields) {
f.setAccessible(true); // expose private fields
final Class type = f.getType();
if (type.isAssignableFrom(context().getClass())) {
// populate annotated Context field
ClassUtils.setValue(f, service, getContext());
continue;
}
if (!Service.class.isAssignableFrom(type)) {
final String error = "Invalid parameter: " +
f.getDeclaringClass().getName() + "#" + f.getName();
if (strict) throw new IllegalArgumentException(error);
log.error(error);
continue;
}
@SuppressWarnings("unchecked")
final Class serviceType =
(Class) type;
Service s = context().getService(serviceType);
if (s == null) {
// recursively obtain needed service
final boolean required = f.getAnnotation(Parameter.class).required();
s = loadService(serviceType, required);
// NB: Remember when there is an optional EventService parameter.
if (s instanceof EventService) eventServiceRequired = required;
}
ClassUtils.setValue(f, service, s);
}
// check for event handlers
if (!ClassUtils.getAnnotatedMethods(c, EventHandler.class).isEmpty()) {
// NB: There are @EventHandler methods; we need an EventService.
loadService(EventService.class, eventServiceRequired);
}
service.initialize();
service.registerEventHandlers();
return service;
}
/** Asks the plugin index for all available service implementations. */
private void findServiceClasses(
final Map, Double> serviceMap,
final List> serviceList)
{
// ask the plugin index for the (sorted) list of available services
final List> services =
context().getPluginIndex().getPlugins(Service.class);
for (final PluginInfo info : services) {
try {
final Class c = info.loadClass();
final double priority = info.getPriority();
serviceMap.put(c, priority);
serviceList.add(c);
}
catch (final Throwable e) {
log.error("Invalid service: " + info, e);
}
}
}
/** Returns true iff the given class is {@link Optional}. */
private boolean isOptional(final Class c) {
return Optional.class.isAssignableFrom(c);
}
}