Cleanup source implementations, and properly cache URI-based sources.

core/src/main/java/gov/nist/secauto/metaschema/core/model/constraint/impl/ExternalSource.java

@@ -58,10 +58,7 @@ public final class ExternalSource implements ISource {
   public static ISource instance(@NonNull URI location) {
     ISource retval;
     synchronized (sources) {
-      retval = sources.get(location);
-      if (retval == null) {
-        retval = new ExternalModelSource(location);
-      }
+      retval = sources.computeIfAbsent(location, (uri) -> new ExternalSource(uri));
     }
     return retval;
   }
@@ -80,4 +77,9 @@ public SourceType getSourceType() {
   public URI getSource() {
     return modelUri;
   }
+
+  @Override
+  public String toString() {
+    return "external:" + modelUri;
+  }
 }

core/src/main/java/gov/nist/secauto/metaschema/core/model/constraint/impl/InternalModelSource.java

@@ -41,7 +41,7 @@
  */
 public final class InternalModelSource implements ISource {
   @NonNull
-  private static final Map<URI, ExternalModelSource> sources = new HashMap<>(); // NOPMD - intentional
+  private static final Map<URI, InternalModelSource> sources = new HashMap<>(); // NOPMD - intentional
   @NonNull
   private final URI modelUri;
 
@@ -57,10 +57,7 @@ public final class InternalModelSource implements ISource {
   public static ISource instance(@NonNull URI location) {
     ISource retval;
     synchronized (sources) {
-      retval = sources.get(location);
-      if (retval == null) {
-        retval = new InternalModelSource(location);
-      }
+      retval = sources.computeIfAbsent(location, (uri) -> new InternalModelSource(uri));
     }
     return retval;
   }
@@ -79,4 +76,9 @@ public SourceType getSourceType() {
   public URI getSource() {
     return modelUri;
   }
+
+  @Override
+  public String toString() {
+    return "internal:" + modelUri;
+  }
 }

core/src/main/java/gov/nist/secauto/metaschema/core/model/constraint/impl/UnknownInternalModelSource.java

@@ -68,4 +68,9 @@ public URI getSource() {
     // always null
     return null;
   }
+
+  @Override
+  public String toString() {
+    return "internal";
+  }
 }