Updates:

-Removed LinkInstPair
-Added installer field to FileOrLink
-Changed to file system in FileOrLink
-Added filter for multiple types in getRelations

src/main/java/ley/anvil/addonscript/wrapper/ASWrapper.java

@@ -27,6 +27,8 @@ public class ASWrapper {
     public Map<String, IInstaller> INSTALLERS;
     public Map<String, AddonscriptJSON> ADDONS;
     public Map<Integer, ASWrapper.VersionWrapper> VERSIONS;
+    @Nullable
+    File asdir;
 
     public ASWrapper(AddonscriptJSON json) {
         this.json = json;
@@ -57,6 +59,7 @@ public class ASWrapper {
 
     public ASWrapper(File file) throws FileNotFoundException {
         this(AddonscriptJSON.read(new FileReader(file)));
+        asdir = file.getParentFile();
     }
 
     public ASWrapper(URL url) {
@@ -122,7 +125,7 @@ public class ASWrapper {
             return version != null;
         }
 
-        public List<RelationWrapper> getRelations(String[] conditions, @Nullable String type) {
+        public List<RelationWrapper> getRelations(String[] conditions, @Nullable String[] types) {
             List<RelationWrapper> list = new ArrayList<>();
             if (exists() && version.relations != null) {
                 for (AddonscriptJSON.Relation r : version.relations) {
@@ -131,7 +134,7 @@ public class ASWrapper {
                         opt = r.options;
                     else
                         opt = defaultOptions();
-                    if (opt.containsAll(Arrays.asList(conditions)) && (type== null || r.type.equals(type))) {
+                    if (opt.containsAll(Arrays.asList(conditions)) && (types == null || Arrays.asList(types).contains(r.type))) {
                         list.add(new RelationWrapper(r));
                     }
                 }
@@ -196,7 +199,7 @@ public class ASWrapper {
         }
 
         public FileOrLink get() {
-            return new FileOrLink(getLink());
+            return new FileOrLink(getLink(), file.installer).setASDir(asdir);
         }
 
         public String getLink() {

src/main/java/ley/anvil/addonscript/wrapper/FileOrLink.java

@@ -10,11 +10,19 @@ import java.net.URL;
 public class FileOrLink {
 
     String link;
+    File asdir;
+    public String installer;
+
 
     public FileOrLink(String link) {
         this.link = link;
     }
 
+    public FileOrLink(String link, String installer) {
+        this.link = link;
+        this.installer = installer;
+    }
+
     public boolean isFile() {
         return Utils.notEmpty(link) && link.startsWith("file://");
     }
@@ -33,12 +41,23 @@ public class FileOrLink {
      * @param path The path to the directory, in which the Addonscript.json is placed
      * @return A File
      */
+    @Deprecated
     public File getFile(@Nonnull String path) {
         if (isFile())
             return new File(Utils.slashEnd(path) + link.replace("file://", ""));
         throw new RuntimeException("This is no file");
     }
 
+    /**
+     * To call this, the ASDir should be set. If it isn't set, set it with setASDir before calling this.
+     * @return The File, which is represented by this object
+     */
+    public File getFile() {
+        if (isFile() && isASDirSet())
+            return new File(Utils.slashEnd(asdir.getPath()) + link.replace("file://", ""));
+        throw new RuntimeException("This is no file or the AS Dir was not set");
+    }
+
     public URL getURL() {
         if (isURL()) {
             try {
@@ -50,4 +69,19 @@ public class FileOrLink {
         throw new RuntimeException("This is no URL");
     }
 
+    /**
+     * In Addonscript file:// links are relative paths from the Addonscript.json,
+     * so you have to specify the path of the directory in which it is placed.
+     * @param dir The path to the directory, in which the Addonscript.json is placed
+     * @return This FileOrLink object
+     */
+    public FileOrLink setASDir(File dir) {
+        asdir = dir;
+        return this;
+    }
+
+    public boolean isASDirSet() {
+        return asdir != null;
+    }
+
 }

src/main/java/ley/anvil/addonscript/wrapper/LinkInstPair.java

@@ -1,52 +0,0 @@
-package ley.anvil.addonscript.wrapper;
-
-import java.io.File;
-import java.net.MalformedURLException;
-import java.net.URL;
-
-public class LinkInstPair {
-
-    public String link;
-    public String installer;
-
-    public LinkInstPair(String link, String installer) {
-        this.link = link;
-        this.installer = installer;
-    }
-
-    public String getLink() {
-        return link;
-    }
-
-    public String getInstaller() {
-        return installer;
-    }
-
-    public boolean isURL() {
-        return link.startsWith("http://") || link.startsWith("https://");
-    }
-
-    public boolean isFile() {
-        return link.startsWith("file://");
-    }
-
-    public File asFile(String jsonDir) {
-        if (isFile()) {
-            String f = link.replace("file://", jsonDir + (jsonDir.endsWith("/") ? "" : "/"));
-            return new File(f);
-        }
-        throw new RuntimeException("This is not a file");
-    }
-
-    public URL asURL() {
-        if (isURL()) {
-            try {
-                return new URL(link);
-            } catch (MalformedURLException e) {
-                throw new RuntimeException(e);
-            }
-        }
-        throw new RuntimeException("This is not a URL");
-    }
-
-}