diff --git a/src/LanguageServer.php b/src/LanguageServer.php
index 118dd937..62452d1e 100644
--- a/src/LanguageServer.php
+++ b/src/LanguageServer.php
@@ -9,7 +9,8 @@
     TextDocumentSyncKind,
     Message,
     InitializeResult,
-    CompletionOptions
+    CompletionOptions,
+    WorkspaceFolder
 };
 use LanguageServer\FilesFinder\{FilesFinder, ClientFilesFinder, FileSystemFilesFinder};
 use LanguageServer\ContentRetriever\{ContentRetriever, ClientContentRetriever, FileSystemContentRetriever};
@@ -161,12 +162,26 @@ public function __construct(ProtocolReader $reader, ProtocolWriter $writer)
      *
      * @param ClientCapabilities $capabilities The capabilities provided by the client (editor)
      * @param string|null $rootPath The rootPath of the workspace. Is null if no folder is open.
+     * @param string|null $rootUri TThe rootUri of the workspace. Is null if no folder is open. If both `rootPath` and `rootUri` are set `rootUri` wins.
+     * @param WorkspaceFolder[]|null $workspaceFolders The actual configured workspace folders.
      * @param int|null $processId The process Id of the parent process that started the server. Is null if the process has not been started by another process. If the parent process is not alive then the server should exit (see exit notification) its process.
      * @return Promise <InitializeResult>
      */
-    public function initialize(ClientCapabilities $capabilities, string $rootPath = null, int $processId = null): Promise
+    public function initialize(ClientCapabilities $capabilities, string $rootPath = null, string $rootUri = null, array $workspaceFolders = null, int $processId = null): Promise
     {
-        return coroutine(function () use ($capabilities, $rootPath, $processId) {
+        return coroutine(function () use ($capabilities, $rootPath, $rootUri, $workspaceFolders, $processId) {
+
+            /** @var string[] */
+            $rootPaths = [];
+            if ($workspaceFolders !== null) {
+                foreach ($workspaceFolders as $workspaceFolder) {
+                    $rootPaths[] = uriToPath($workspaceFolder->uri);
+                }
+            } else if ($rootUri !== null) {
+                $rootPaths[] = uriToPath($rootUri);
+            } else if ($rootPath !== null) {
+                $rootPaths[] = $rootPath;
+            }
 
             if ($capabilities->xfilesProvider) {
                 $this->filesFinder = new ClientFilesFinder($this->client);
@@ -199,23 +214,23 @@ public function initialize(ClientCapabilities $capabilities, string $rootPath =
                 yield $this->beforeIndex($rootPath);
 
                 // Find composer.json
-                if ($this->composerJson === null) {
-                    $composerJsonFiles = yield $this->filesFinder->find(Path::makeAbsolute('**/composer.json', $rootPath));
-                    sortUrisLevelOrder($composerJsonFiles);
-
-                    if (!empty($composerJsonFiles)) {
-                        $this->composerJson = json_decode(yield $this->contentRetriever->retrieve($composerJsonFiles[0]));
-                    }
+                if ($this->composerJsons === null) {
+                    $this->composerJsons = yield array_map(function (string $rootPath) {
+                        $composerJsonFiles = yield $this->filesFinder->find(Path::makeAbsolute('**/composer.json', $rootPath));
+                        foreach ($composerJsonFiles as $composerJsonFile) {
+                            $this->composerJsons[$composerJsonFile] = json_decode(yield $this->contentRetriever->retrieve($composerJsonFile));
+                        }
+                    }, $rootPaths);
                 }
 
                 // Find composer.lock
-                if ($this->composerLock === null) {
-                    $composerLockFiles = yield $this->filesFinder->find(Path::makeAbsolute('**/composer.lock', $rootPath));
-                    sortUrisLevelOrder($composerLockFiles);
-
-                    if (!empty($composerLockFiles)) {
-                        $this->composerLock = json_decode(yield $this->contentRetriever->retrieve($composerLockFiles[0]));
-                    }
+                if ($this->composerLocks === null) {
+                    $this->composerLocks = yield array_map(function (string $rootPath) {
+                        $composerLockFiles = yield $this->filesFinder->find(Path::makeAbsolute('**/composer.lock', $rootPath));
+                        foreach ($composerLockFiles as $composerLockFile) {
+                            $this->composerLocks[$composerLockFile] = json_decode(yield $this->contentRetriever->retrieve($composerLockFile));
+                        }
+                    }, $rootPaths);
                 }
 
                 $cache = $capabilities->xcacheProvider ? new ClientCache($this->client) : new FileSystemCache;
@@ -229,8 +244,8 @@ public function initialize(ClientCapabilities $capabilities, string $rootPath =
                     $dependenciesIndex,
                     $sourceIndex,
                     $this->documentLoader,
-                    $this->composerLock,
-                    $this->composerJson
+                    $this->composerLocks,
+                    $this->composerJsons
                 );
                 $indexer->index()->otherwise('\\LanguageServer\\crash');
             }
@@ -242,8 +257,8 @@ public function initialize(ClientCapabilities $capabilities, string $rootPath =
                     $this->definitionResolver,
                     $this->client,
                     $this->globalIndex,
-                    $this->composerJson,
-                    $this->composerLock
+                    $this->composerJsons,
+                    $this->composerLocks
                 );
             }
             if ($this->workspace === null) {
@@ -252,9 +267,9 @@ public function initialize(ClientCapabilities $capabilities, string $rootPath =
                     $this->projectIndex,
                     $dependenciesIndex,
                     $sourceIndex,
-                    $this->composerLock,
+                    $this->composerLocks,
                     $this->documentLoader,
-                    $this->composerJson
+                    $this->composerJsons
                 );
             }
 
diff --git a/src/Protocol/WorkspaceFolder.php b/src/Protocol/WorkspaceFolder.php
new file mode 100644
index 00000000..535b8c07
--- /dev/null
+++ b/src/Protocol/WorkspaceFolder.php
@@ -0,0 +1,20 @@
+<?php
+
+namespace LanguageServer\Protocol;
+
+class WorkspaceFolder
+{
+    /**
+	 * The associated URI for this workspace folder.
+     *
+	 * @var string
+	 */
+	public $uri;
+
+	/**
+	 * The name of the workspace folder. Defaults to the uri's basename.
+     *
+	 * @var string
+	 */
+	public $name;
+}