add descriptor loader

protobuf/pom.xml

@@ -47,6 +47,10 @@ abstractions.
       <artifactId>jackson-annotations</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>com.fasterxml.jackson.core</groupId>
+      <artifactId>jackson-annotations</artifactId>
+    </dependency>
   </dependencies>
 
   <build>

protobuf/src/main/java/com/fasterxml/jackson/dataformat/protobuf/schema/DescriptorLoader.java

@@ -0,0 +1,82 @@
+package com.fasterxml.jackson.dataformat.protobuf.schema;
+
+
+import com.fasterxml.jackson.dataformat.protobuf.ProtobufMapper;
+
+import java.io.ByteArrayInputStream;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.URL;
+
+/**
+ * Class used for loading protobuf descriptors (from .desc files
+ * or equivalent sources), to construct FileDescriptorSet.
+ */
+public class DescriptorLoader
+{
+    private final String DESCRIPTOR_PROTO = "/descriptor.proto";
+    private ProtobufMapper descriptorMapper;
+    private ProtobufSchema descriptorFileSchema;
+
+    /**
+     * Standard loader instance that is usually used for loading descriptor file.
+     */
+    public final static DescriptorLoader std = new DescriptorLoader();
+
+    public DescriptorLoader() {}
+
+
+    /**
+     * Public API
+     */
+
+    public FileDescriptorSet load(URL url) throws IOException
+    {
+        return _loadFileDescriptorSet(url.openStream());
+    }
+
+    public FileDescriptorSet load(File f) throws IOException
+    {
+        return _loadFileDescriptorSet(new FileInputStream(f));
+    }
+
+    public FileDescriptorSet load(InputStream in) throws IOException
+    {
+        return _loadFileDescriptorSet(in);
+    }
+
+    public FileDescriptorSet fromBytes(byte[] descriptorBytes) throws IOException
+    {
+        return _loadFileDescriptorSet(new ByteArrayInputStream(descriptorBytes));
+    }
+
+    protected FileDescriptorSet _loadFileDescriptorSet(InputStream in) throws IOException
+    {
+        try {
+            if (descriptorMapper == null) {
+                createDescriptorMapper();
+            }
+            return descriptorMapper.readerFor(FileDescriptorSet.class)
+                                   .with(descriptorFileSchema)
+                                   .readValue(in);
+        }
+        finally {
+            try {
+                in.close();
+            }
+            catch (IOException e) {
+            }
+        }
+    }
+
+    private void createDescriptorMapper() throws IOException
+    {
+        // read Descriptor Proto
+        descriptorMapper = new ProtobufMapper();
+        InputStream in = getClass().getResourceAsStream(DESCRIPTOR_PROTO);
+        descriptorFileSchema = ProtobufSchemaLoader.std.load(in, "FileDescriptorSet");
+        in.close();
+    }
+}