Adding MSICredentails type (Azure#250)

* Adding MSICredentails type

* Moving adapter as MSICredentails memeber variable

azure-client-authentication/src/main/java/com/microsoft/azure/credentials/MSICredentials.java

@@ -0,0 +1,102 @@
+/**
+ * Copyright (c) Microsoft Corporation. All rights reserved.
+ * Licensed under the MIT License. See License.txt in the project root for
+ * license information.
+ */
+
+package com.microsoft.azure.credentials;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.microsoft.azure.AzureEnvironment;
+import com.microsoft.azure.management.apigeneration.Beta;
+import com.microsoft.azure.serializer.AzureJacksonAdapter;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.OutputStreamWriter;
+import java.net.HttpURLConnection;
+import java.net.URL;
+
+/**
+ * Managed Service Identity token based credentials for use with a REST Service Client.
+ */
+@Beta
+public class MSICredentials extends AzureTokenCredentials {
+    private final String resource;
+    private final int msiPort;
+    private final AzureJacksonAdapter adapter;
+    /**
+     * Initializes a new instance of the MSICredentials.
+     *
+     * @param environment the Azure environment to use
+     */
+    public MSICredentials(AzureEnvironment environment) {
+        this(environment, 50342);
+    }
+
+    /**
+     * Initializes a new instance of the MSICredentials.
+     *
+     * @param environment the Azure environment to use
+     * @param msiPort the local port to retrieve token from
+     */
+    public MSICredentials(AzureEnvironment environment, int msiPort) {
+        super(environment, null /** retrieving MSI token does not require tenant **/);
+        this.resource = environment.resourceManagerEndpoint();
+        this.msiPort = msiPort;
+        this.adapter = new AzureJacksonAdapter();
+    }
+
+    @Override
+    public String getToken(String resource) throws IOException {
+        URL url = new URL(String.format("http://localhost:%d/oauth2/token", this.msiPort));
+        String postData = String.format("resource=%s", this.resource);
+        HttpURLConnection connection = null;
+
+        try {
+            connection = (HttpURLConnection) url.openConnection();
+
+            connection.setRequestMethod("POST");
+            connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded; charset=utf-8");
+            connection.setRequestProperty("Metadata", "true");
+            connection.setRequestProperty("Content-Length", Integer.toString(postData.length()));
+            connection.setDoOutput(true);
+
+            connection.connect();
+
+            OutputStreamWriter wr = new OutputStreamWriter(connection.getOutputStream());
+            wr.write(postData);
+            wr.flush();
+
+            InputStream stream = connection.getInputStream();
+            BufferedReader reader = new BufferedReader(new InputStreamReader(stream, "UTF-8"), 100);
+            String result = reader.readLine();
+
+            MSIToken msiToken = adapter.deserialize(result, MSIToken.class);
+            return msiToken.accessToken;
+        } finally {
+            if (connection != null) {
+                connection.disconnect();
+            }
+        }
+    }
+
+    /**
+     * Type representing response from the local MSI token provider.
+     */
+    private static class MSIToken {
+        /**
+         * Token type "Bearer".
+         */
+        @JsonProperty(value = "token_type")
+        private String tokenType;
+
+        /**
+         * Access token.
+         */
+        @JsonProperty(value = "access_token")
+        private String accessToken;
+    }
+}