Context: allow multi-injection in non-strict mode

With scijava.context.strict set to false (i.e.: strict=false flag of the
Context), injecting an object multiple times is now allowed.

This avoids the dreaded "Context already injected" exception which is
thrown, for example, when reusing an op with a second module.

This commit is dedicated to Christian Dietz!

Author: ctrueden

src/main/java/org/scijava/Context.java

@@ -436,7 +436,7 @@ private void inject(final Field f, final Object o) {
 			final Class<?> type = f.getType();
 			if (Service.class.isAssignableFrom(type)) {
 				final Service existingService = (Service) ClassUtils.getValue(f, o);
-				if (existingService != null) {
+				if (strict && existingService != null) {
 					throw new IllegalStateException("Context already injected: " +
 							f.getDeclaringClass().getName() + "#" + f.getName());
 				}
@@ -450,14 +450,24 @@ private void inject(final Field f, final Object o) {
 					throw new IllegalArgumentException(
 						createMissingServiceMessage(serviceType));
 				}
+				if (existingService != null && existingService != service) {
+					// NB: Can only happen in non-strict mode.
+					throw new IllegalStateException("Mismatched context: " +
+							f.getDeclaringClass().getName() + "#" + f.getName());
+				}
 				ClassUtils.setValue(f, o, service);
 			}
 			else if (Context.class.isAssignableFrom(type) && type.isInstance(this)) {
 				final Context existingContext = (Context) ClassUtils.getValue(f, o);
-				if (existingContext != null) {
+				if (strict && existingContext != null) {
 					throw new IllegalStateException("Context already injected: " +
 							f.getDeclaringClass().getName() + "#" + f.getName());
 				}
+				if (existingContext != null && existingContext != this) {
+					// NB: Can only happen in non-strict mode.
+					throw new IllegalStateException("Mismatched context: " +
+							f.getDeclaringClass().getName() + "#" + f.getName());
+				}
 
 				// populate Context parameter
 				ClassUtils.setValue(f, o, this);