added a JemClassLoader which can "add" jem on the fly

src/main/java/com/github/jrubygradle/jem/Gem.java

@@ -7,7 +7,6 @@ import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.dataformat.yaml.YAMLFactory;
 
 import java.io.File;
-import java.io.IOError;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.List;

src/main/java/com/github/jrubygradle/jem/JemsClassLoader.java

@@ -0,0 +1,50 @@
+package com.github.jrubygradle.jem;
+
+import com.github.jrubygradle.jem.internal.GemInstaller;
+import com.github.jrubygradle.jem.GemInstaller.DuplicateBehavior;
+
+import java.io.File;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.file.Files;
+import java.util.Collections;
+
+/**
+ * Created by cmeier on 8/15/15.
+ */
+public class JemsClassLoader extends URLClassLoader {
+
+    private final File jemsDir;
+
+    public static JemsClassLoader create() throws IOException {
+        return new JemsClassLoader(Files.createTempDirectory("jems").toFile());
+    }
+
+    public JemsClassLoader(File jemsDir) throws IOException {
+        super(new URL[] { jemsDir.toURI().toURL() });
+        this.jemsDir = jemsDir;
+    }
+
+    public Gem addJem(String jem) throws IOException {
+        return addJem(new File(jem));
+    }
+    public Gem addJem(File jem) throws IOException {
+        GemInstaller installer = new GemInstaller(jemsDir.getAbsolutePath(), Collections.<File>emptyList());
+        installer.mkdirs();
+        Gem gemspec = installer.installGem(jemsDir, jem, DuplicateBehavior.OVERWRITE);
+        String dir = "gems/" + installer.gemFullName(gemspec);
+        createDirInfoFile(dir);
+        for(String path: gemspec.requirePaths){
+            createDirInfoFile(dir + "/" + path);
+        }
+        return gemspec;
+    }
+
+    private void createDirInfoFile(String dir) throws IOException {
+        try (FileWriter out = new FileWriter(new File(jemsDir, dir + "/.jrubydir"))){
+            out.append(".");
+        }
+    }
+}

src/main/java/com/github/jrubygradle/jem/internal/GemInstaller.java

@@ -50,14 +50,14 @@ public class GemInstaller {
         }
     }
 
-    public boolean installGem(File installDir, File gem, DuplicateBehavior onDuplicate) {
+    public Gem installGem(File installDir, File gem, DuplicateBehavior onDuplicate) {
         /* TODO: isValidGem? */
         try {
             cacheGemInInstallDir(installDir, gem);
         }
         catch (IOException ex) {
             logger.error("Failed to cache our gem in %s", installDir, ex);
-            return false;
+            return null;
         }
 
         Gem gemMetadata;
@@ -73,7 +73,7 @@ public class GemInstaller {
         }
         catch (IOException ex) {
             logger.error("Failed to process the metadata", ex);
-            return false;
+            return null;
         }
         logger.info(String.format("We've processed metadata for %s at version %s",
                 gemMetadata.name, gemMetadata.version.version));
@@ -96,7 +96,7 @@ public class GemInstaller {
                     gemMetadata.name, installDir), ex);
         }
 
-        return true;
+        return gemMetadata;
     }
 
     /**

src/test/groovy/com/github/jrubygradle/jem/internal/GemInstallerSpec.groovy

@@ -6,7 +6,6 @@ import spock.lang.*
 
 import java.nio.file.Files
 import java.nio.file.Path
-import org.apache.commons.io.FileUtils
 
 class GemInstallerSpec extends Specification {
     static final String FIXTURES_ROOT = new File(['src', 'test', 'resources'].join(File.separator)).absolutePath