diff --git a/src/Composer/Command/InstallCommand.php b/src/Composer/Command/InstallCommand.php
index b992758bb..df1edf062 100644
--- a/src/Composer/Command/InstallCommand.php
+++ b/src/Composer/Command/InstallCommand.php
@@ -98,7 +98,7 @@ EOT
 
         $composer = $this->requireComposer();
 
-        if ((!$composer->getLocker() || !$composer->getLocker()->isLocked()) && !HttpDownloader::isCurlEnabled()) {
+        if (!$composer->getLocker()->isLocked() && !HttpDownloader::isCurlEnabled()) {
             $io->writeError('<warning>Composer is operating significantly slower than normal because you do not have the PHP curl extension enabled.</warning>');
         }
 
diff --git a/src/Composer/Composer.php b/src/Composer/Composer.php
index 6bdc54908..8913397b6 100644
--- a/src/Composer/Composer.php
+++ b/src/Composer/Composer.php
@@ -12,15 +12,10 @@
 
 namespace Composer;
 
-use Composer\Package\RootPackageInterface;
 use Composer\Package\Locker;
 use Composer\Pcre\Preg;
-use Composer\Util\Loop;
-use Composer\Repository\RepositoryManager;
-use Composer\Installer\InstallationManager;
 use Composer\Plugin\PluginManager;
 use Composer\Downloader\DownloadManager;
-use Composer\EventDispatcher\EventDispatcher;
 use Composer\Autoload\AutoloadGenerator;
 use Composer\Package\Archiver\ArchiveManager;
 
@@ -29,7 +24,7 @@ use Composer\Package\Archiver\ArchiveManager;
  * @author Konstantin Kudryashiv <ever.zet@gmail.com>
  * @author Nils Adermann <naderman@naderman.de>
  */
-class Composer
+class Composer extends PartialComposer
 {
     /*
      * Examples of the following constants in the various configurations they can be in
@@ -87,220 +82,76 @@ class Composer
     }
 
     /**
-     * @var RootPackageInterface
+     * @var Locker
      */
-    private $package;
+    private $locker;
 
     /**
-     * @var Locker|null
+     * @var Downloader\DownloadManager
      */
-    private $locker = null;
+    private $downloadManager;
 
     /**
-     * @var Loop
+     * @var Plugin\PluginManager
      */
-    private $loop;
+    private $pluginManager;
 
     /**
-     * @var Repository\RepositoryManager
+     * @var Autoload\AutoloadGenerator
      */
-    private $repositoryManager;
+    private $autoloadGenerator;
 
     /**
-     * @var Downloader\DownloadManager|null
+     * @var ArchiveManager
      */
-    private $downloadManager = null;
+    private $archiveManager;
 
-    /**
-     * @var Installer\InstallationManager
-     */
-    private $installationManager;
-
-    /**
-     * @var Plugin\PluginManager|null
-     */
-    private $pluginManager = null;
-
-    /**
-     * @var Config
-     */
-    private $config;
-
-    /**
-     * @var EventDispatcher
-     */
-    private $eventDispatcher;
-
-    /**
-     * @var Autoload\AutoloadGenerator|null
-     */
-    private $autoloadGenerator = null;
-
-    /**
-     * @var ArchiveManager|null
-     */
-    private $archiveManager = null;
-
-    /**
-     * @return void
-     */
-    public function setPackage(RootPackageInterface $package): void
-    {
-        $this->package = $package;
-    }
-
-    /**
-     * @return RootPackageInterface
-     */
-    public function getPackage(): RootPackageInterface
-    {
-        return $this->package;
-    }
-
-    /**
-     * @return void
-     */
-    public function setConfig(Config $config): void
-    {
-        $this->config = $config;
-    }
-
-    /**
-     * @return Config
-     */
-    public function getConfig(): Config
-    {
-        return $this->config;
-    }
-
-    /**
-     * @return void
-     */
     public function setLocker(Locker $locker): void
     {
         $this->locker = $locker;
     }
 
-    /**
-     * @return ?Locker
-     */
-    public function getLocker(): ?Locker
+    public function getLocker(): Locker
     {
         return $this->locker;
     }
 
-    /**
-     * @return void
-     */
-    public function setLoop(Loop $loop): void
-    {
-        $this->loop = $loop;
-    }
-
-    /**
-     * @return Loop
-     */
-    public function getLoop(): Loop
-    {
-        return $this->loop;
-    }
-
-    /**
-     * @return void
-     */
-    public function setRepositoryManager(RepositoryManager $manager): void
-    {
-        $this->repositoryManager = $manager;
-    }
-
-    /**
-     * @return RepositoryManager
-     */
-    public function getRepositoryManager(): RepositoryManager
-    {
-        return $this->repositoryManager;
-    }
-
-    /**
-     * @return void
-     */
     public function setDownloadManager(DownloadManager $manager): void
     {
         $this->downloadManager = $manager;
     }
 
-    public function getDownloadManager(): ?DownloadManager
+    public function getDownloadManager(): DownloadManager
     {
         return $this->downloadManager;
     }
 
-    /**
-     * @return void
-     */
     public function setArchiveManager(ArchiveManager $manager): void
     {
         $this->archiveManager = $manager;
     }
 
-    public function getArchiveManager(): ?ArchiveManager
+    public function getArchiveManager(): ArchiveManager
     {
         return $this->archiveManager;
     }
 
-    /**
-     * @return void
-     */
-    public function setInstallationManager(InstallationManager $manager): void
-    {
-        $this->installationManager = $manager;
-    }
-
-    /**
-     * @return InstallationManager
-     */
-    public function getInstallationManager(): InstallationManager
-    {
-        return $this->installationManager;
-    }
-
-    /**
-     * @return void
-     */
     public function setPluginManager(PluginManager $manager): void
     {
         $this->pluginManager = $manager;
     }
 
-    public function getPluginManager(): ?PluginManager
+    public function getPluginManager(): PluginManager
     {
         return $this->pluginManager;
     }
 
-    /**
-     * @return void
-     */
-    public function setEventDispatcher(EventDispatcher $eventDispatcher): void
-    {
-        $this->eventDispatcher = $eventDispatcher;
-    }
-
-    /**
-     * @return EventDispatcher
-     */
-    public function getEventDispatcher(): EventDispatcher
-    {
-        return $this->eventDispatcher;
-    }
-
-    /**
-     * @return void
-     */
     public function setAutoloadGenerator(AutoloadGenerator $autoloadGenerator): void
     {
         $this->autoloadGenerator = $autoloadGenerator;
     }
 
-    public function getAutoloadGenerator(): ?AutoloadGenerator
+    public function getAutoloadGenerator(): AutoloadGenerator
     {
         return $this->autoloadGenerator;
     }
diff --git a/src/Composer/Console/Application.php b/src/Composer/Console/Application.php
index 24811b34b..0d1b19ce1 100644
--- a/src/Composer/Console/Application.php
+++ b/src/Composer/Console/Application.php
@@ -560,7 +560,7 @@ class Application extends BaseApplication
 
         $composer = $this->getComposer(false, false);
         if (null === $composer) {
-            $composer = Factory::createGlobal($this->io);
+            $composer = Factory::createGlobal($this->io, $this->disablePluginsByDefault, $this->disableScriptsByDefault);
         }
 
         if (null !== $composer) {
diff --git a/src/Composer/EventDispatcher/EventDispatcher.php b/src/Composer/EventDispatcher/EventDispatcher.php
index 704c1e403..ea6d51792 100644
--- a/src/Composer/EventDispatcher/EventDispatcher.php
+++ b/src/Composer/EventDispatcher/EventDispatcher.php
@@ -12,10 +12,12 @@
 
 namespace Composer\EventDispatcher;
 
+use Composer\Autoload\AutoloadGenerator;
 use Composer\DependencyResolver\Transaction;
 use Composer\Installer\InstallerEvent;
 use Composer\IO\IOInterface;
 use Composer\Composer;
+use Composer\PartialComposer;
 use Composer\Pcre\Preg;
 use Composer\Util\Platform;
 use Composer\DependencyResolver\Operation\OperationInterface;
@@ -44,7 +46,7 @@ use Symfony\Component\Process\ExecutableFinder;
  */
 class EventDispatcher
 {
-    /** @var Composer */
+    /** @var PartialComposer */
     protected $composer;
     /** @var IOInterface */
     protected $io;
@@ -62,11 +64,11 @@ class EventDispatcher
     /**
      * Constructor.
      *
-     * @param Composer        $composer The composer instance
+     * @param PartialComposer $composer The composer instance
      * @param IOInterface     $io       The IOInterface instance
      * @param ProcessExecutor $process
      */
-    public function __construct(Composer $composer, IOInterface $io, ProcessExecutor $process = null)
+    public function __construct(PartialComposer $composer, IOInterface $io, ProcessExecutor $process = null)
     {
         $this->composer = $composer;
         $this->io = $io;
@@ -116,6 +118,8 @@ class EventDispatcher
      */
     public function dispatchScript($eventName, $devMode = false, $additionalArgs = array(), $flags = array())
     {
+        assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
+
         return $this->doDispatch(new Script\Event($eventName, $this->composer, $this->io, $devMode, $additionalArgs, $flags));
     }
 
@@ -133,6 +137,8 @@ class EventDispatcher
      */
     public function dispatchPackageEvent($eventName, $devMode, RepositoryInterface $localRepo, array $operations, OperationInterface $operation)
     {
+        assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
+
         return $this->doDispatch(new PackageEvent($eventName, $this->composer, $this->io, $devMode, $localRepo, $operations, $operation));
     }
 
@@ -149,6 +155,8 @@ class EventDispatcher
      */
     public function dispatchInstallerEvent($eventName, $devMode, $executeOperations, Transaction $transaction)
     {
+        assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
+
         return $this->doDispatch(new InstallerEvent($eventName, $this->composer, $this->io, $devMode, $executeOperations, $transaction));
     }
 
@@ -486,6 +494,8 @@ class EventDispatcher
             return array();
         }
 
+        assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
+
         if ($this->loader) {
             $this->loader->unregister();
         }
diff --git a/src/Composer/Factory.php b/src/Composer/Factory.php
index 4f7c6fb2d..e6ef47563 100644
--- a/src/Composer/Factory.php
+++ b/src/Composer/Factory.php
@@ -28,6 +28,7 @@ use Composer\Util\Loop;
 use Composer\Util\Silencer;
 use Composer\Plugin\PluginEvents;
 use Composer\EventDispatcher\Event;
+use Phar;
 use Seld\JsonLint\DuplicateKeyException;
 use Symfony\Component\Console\Formatter\OutputFormatter;
 use Symfony\Component\Console\Formatter\OutputFormatterStyle;
@@ -39,6 +40,7 @@ use Composer\Downloader\TransportException;
 use Composer\Json\JsonValidationException;
 use Composer\Repository\InstalledRepositoryInterface;
 use Seld\JsonLint\JsonParser;
+use ZipArchive;
 
 /**
  * Creates a configured instance of composer.
@@ -52,9 +54,8 @@ class Factory
 {
     /**
      * @throws \RuntimeException
-     * @return string
      */
-    protected static function getHomeDir()
+    protected static function getHomeDir(): string
     {
         $home = Platform::getEnv('COMPOSER_HOME');
         if ($home) {
@@ -95,11 +96,7 @@ class Factory
         return $dirs[0];
     }
 
-    /**
-     * @param  string $home
-     * @return string
-     */
-    protected static function getCacheDir($home)
+    protected static function getCacheDir(string $home): string
     {
         $cacheDir = Platform::getEnv('COMPOSER_CACHE_DIR');
         if ($cacheDir) {
@@ -144,11 +141,7 @@ class Factory
         return $home . '/cache';
     }
 
-    /**
-     * @param  string $home
-     * @return string
-     */
-    protected static function getDataDir($home)
+    protected static function getDataDir(string $home): string
     {
         $homeEnv = Platform::getEnv('COMPOSER_HOME');
         if ($homeEnv) {
@@ -169,12 +162,7 @@ class Factory
         return $home;
     }
 
-    /**
-     * @param string|null $cwd
-     *
-     * @return Config
-     */
-    public static function createConfig(IOInterface $io = null, $cwd = null)
+    public static function createConfig(IOInterface $io = null, ?string $cwd = null): Config
     {
         $cwd = $cwd ?: (string) getcwd();
 
@@ -243,20 +231,12 @@ class Factory
         return $config;
     }
 
-    /**
-     * @return string
-     */
-    public static function getComposerFile()
+    public static function getComposerFile(): string
     {
         return trim((string) Platform::getEnv('COMPOSER')) ?: './composer.json';
     }
 
-    /**
-     * @param string $composerFile
-     *
-     * @return string
-     */
-    public static function getLockFile($composerFile)
+    public static function getLockFile(string $composerFile): string
     {
         return "json" === pathinfo($composerFile, PATHINFO_EXTENSION)
                 ? substr($composerFile, 0, -4).'lock'
@@ -266,7 +246,7 @@ class Factory
     /**
      * @return array{highlight: OutputFormatterStyle, warning: OutputFormatterStyle}
      */
-    public static function createAdditionalStyles()
+    public static function createAdditionalStyles(): array
     {
         return array(
             'highlight' => new OutputFormatterStyle('red'),
@@ -274,12 +254,7 @@ class Factory
         );
     }
 
-    /**
-     * Creates a ConsoleOutput instance
-     *
-     * @return ConsoleOutput
-     */
-    public static function createOutput()
+    public static function createOutput(): ConsoleOutput
     {
         $styles = self::createAdditionalStyles();
         $formatter = new OutputFormatter(false, $styles);
@@ -299,9 +274,9 @@ class Factory
      * @param  bool                              $fullLoad       Whether to initialize everything or only main project stuff (used when loading the global composer)
      * @throws \InvalidArgumentException
      * @throws \UnexpectedValueException
-     * @return Composer
+     * @return Composer|PartialComposer Composer if $fullLoad is true, otherwise PartialComposer
      */
-    public function createComposer(IOInterface $io, $localConfig = null, $disablePlugins = false, $cwd = null, $fullLoad = true, $disableScripts = false)
+    public function createComposer(IOInterface $io, $localConfig = null, bool $disablePlugins = false, ?string $cwd = null, bool $fullLoad = true, bool $disableScripts = false)
     {
         $cwd = $cwd ?: (string) getcwd();
 
@@ -363,7 +338,7 @@ class Factory
         $vendorDir = $config->get('vendor-dir');
 
         // initialize composer
-        $composer = new Composer();
+        $composer = $fullLoad ? new Composer() : new PartialComposer();
         $composer->setConfig($config);
 
         if ($fullLoad) {
@@ -410,7 +385,7 @@ class Factory
         $im = $this->createInstallationManager($loop, $io, $dispatcher);
         $composer->setInstallationManager($im);
 
-        if ($fullLoad) {
+        if ($composer instanceof Composer) {
             // initialize download manager
             $dm = $this->createDownloadManager($io, $config, $httpDownloader, $process, $dispatcher);
             $composer->setDownloadManager($dm);
@@ -427,7 +402,7 @@ class Factory
         // add installers to the manager (must happen after download manager is created since they read it out of $composer)
         $this->createDefaultInstallers($im, $composer, $io, $process);
 
-        if ($fullLoad) {
+        if ($composer instanceof Composer) {
             $globalComposer = null;
             if (realpath($config->get('home')) !== $cwd) {
                 $globalComposer = $this->createGlobalComposer($io, $config, $disablePlugins, $disableScripts);
@@ -440,7 +415,7 @@ class Factory
         }
 
         // init locker if possible
-        if ($fullLoad && isset($composerFile)) {
+        if ($composer instanceof Composer && isset($composerFile)) {
             $lockFile = self::getLockFile($composerFile);
 
             $locker = new Package\Locker($io, new JsonFile($lockFile, null, $io), $im, file_get_contents($composerFile), $process);
@@ -460,16 +435,17 @@ class Factory
     }
 
     /**
-     * @param  IOInterface   $io             IO instance
      * @param  bool          $disablePlugins Whether plugins should not be loaded
      * @param  bool          $disableScripts Whether scripts should not be executed
-     * @return Composer|null
      */
-    public static function createGlobal(IOInterface $io, $disablePlugins = false, $disableScripts = false)
+    public static function createGlobal(IOInterface $io, bool $disablePlugins = false, bool $disableScripts = false): ?Composer
     {
         $factory = new static();
 
-        return $factory->createGlobalComposer($io, static::createConfig($io), $disablePlugins, $disableScripts, true);
+        $composer = $factory->createGlobalComposer($io, static::createConfig($io), $disablePlugins, $disableScripts, true);
+        assert(null === $composer || $composer instanceof Composer);
+
+        return $composer;
     }
 
     /**
@@ -478,7 +454,7 @@ class Factory
      *
      * @return void
      */
-    protected function addLocalRepository(IOInterface $io, RepositoryManager $rm, $vendorDir, RootPackageInterface $rootPackage, ProcessExecutor $process = null)
+    protected function addLocalRepository(IOInterface $io, RepositoryManager $rm, $vendorDir, RootPackageInterface $rootPackage, ProcessExecutor $process = null): void
     {
         $fs = null;
         if ($process) {
@@ -489,13 +465,9 @@ class Factory
     }
 
     /**
-     * @param bool $disablePlugins
-     * @param bool $disableScripts
-     * @param bool $fullLoad
-     *
-     * @return Composer|null
+     * @return PartialComposer|Composer|null By default PartialComposer, but Composer if $fullLoad is set to true
      */
-    protected function createGlobalComposer(IOInterface $io, Config $config, $disablePlugins, $disableScripts, $fullLoad = false)
+    protected function createGlobalComposer(IOInterface $io, Config $config, bool $disablePlugins, bool $disableScripts, bool $fullLoad = false): ?PartialComposer
     {
         $composer = null;
         try {
@@ -513,7 +485,7 @@ class Factory
      * @param  EventDispatcher            $eventDispatcher
      * @return Downloader\DownloadManager
      */
-    public function createDownloadManager(IOInterface $io, Config $config, HttpDownloader $httpDownloader, ProcessExecutor $process, EventDispatcher $eventDispatcher = null)
+    public function createDownloadManager(IOInterface $io, Config $config, HttpDownloader $httpDownloader, ProcessExecutor $process, EventDispatcher $eventDispatcher = null): Downloader\DownloadManager
     {
         $cache = null;
         if ($config->get('cache-files-ttl') > 0) {
@@ -566,20 +538,20 @@ class Factory
     public function createArchiveManager(Config $config, Downloader\DownloadManager $dm, Loop $loop)
     {
         $am = new Archiver\ArchiveManager($dm, $loop);
-        $am->addArchiver(new Archiver\ZipArchiver);
-        $am->addArchiver(new Archiver\PharArchiver);
+        if (class_exists(ZipArchive::class)) {
+            $am->addArchiver(new Archiver\ZipArchiver);
+        }
+        if (class_exists(Phar::class)) {
+            $am->addArchiver(new Archiver\PharArchiver);
+        }
 
         return $am;
     }
 
     /**
-     * @param  IOInterface          $io
-     * @param  Composer             $composer
-     * @param  Composer             $globalComposer
-     * @param  bool                 $disablePlugins
      * @return Plugin\PluginManager
      */
-    protected function createPluginManager(IOInterface $io, Composer $composer, Composer $globalComposer = null, $disablePlugins = false)
+    protected function createPluginManager(IOInterface $io, Composer $composer, PartialComposer $globalComposer = null, bool $disablePlugins = false): Plugin\PluginManager
     {
         return new Plugin\PluginManager($io, $composer, $globalComposer, $disablePlugins);
     }
@@ -587,7 +559,7 @@ class Factory
     /**
      * @return Installer\InstallationManager
      */
-    public function createInstallationManager(Loop $loop, IOInterface $io, EventDispatcher $eventDispatcher = null)
+    public function createInstallationManager(Loop $loop, IOInterface $io, EventDispatcher $eventDispatcher = null): Installer\InstallationManager
     {
         return new Installer\InstallationManager($loop, $io, $eventDispatcher);
     }
@@ -595,7 +567,7 @@ class Factory
     /**
      * @return void
      */
-    protected function createDefaultInstallers(Installer\InstallationManager $im, Composer $composer, IOInterface $io, ProcessExecutor $process = null)
+    protected function createDefaultInstallers(Installer\InstallationManager $im, PartialComposer $composer, IOInterface $io, ProcessExecutor $process = null): void
     {
         $fs = new Filesystem($process);
         $binaryInstaller = new Installer\BinaryInstaller($io, rtrim($composer->getConfig()->get('bin-dir'), '/'), $composer->getConfig()->get('bin-compat'), $fs, rtrim($composer->getConfig()->get('vendor-dir'), '/'));
@@ -608,10 +580,8 @@ class Factory
     /**
      * @param InstalledRepositoryInterface   $repo repository to purge packages from
      * @param Installer\InstallationManager  $im   manager to check whether packages are still installed
-     *
-     * @return void
      */
-    protected function purgePackages(InstalledRepositoryInterface $repo, Installer\InstallationManager $im)
+    protected function purgePackages(InstalledRepositoryInterface $repo, Installer\InstallationManager $im): void
     {
         foreach ($repo->getPackages() as $package) {
             if (!$im->isPackageInstalled($repo, $package)) {
@@ -620,10 +590,7 @@ class Factory
         }
     }
 
-    /**
-     * @return Package\Loader\RootPackageLoader
-     */
-    protected function loadRootPackage(RepositoryManager $rm, Config $config, VersionParser $parser, VersionGuesser $guesser, IOInterface $io)
+    protected function loadRootPackage(RepositoryManager $rm, Config $config, VersionParser $parser, VersionGuesser $guesser, IOInterface $io): Package\Loader\RootPackageLoader
     {
         return new Package\Loader\RootPackageLoader($rm, $config, $parser, $guesser, $io);
     }
@@ -636,11 +603,14 @@ class Factory
      * @param  bool        $disableScripts Whether scripts should not be run
      * @return Composer
      */
-    public static function create(IOInterface $io, $config = null, $disablePlugins = false, $disableScripts = false)
+    public static function create(IOInterface $io, $config = null, bool $disablePlugins = false, bool $disableScripts = false): Composer
     {
         $factory = new static();
 
-        return $factory->createComposer($io, $config, $disablePlugins, null, true, $disableScripts);
+        $composer = $factory->createComposer($io, $config, $disablePlugins, null, true, $disableScripts);
+        assert($composer instanceof Composer);
+
+        return $composer;
     }
 
     /**
@@ -651,7 +621,7 @@ class Factory
      * @param  mixed[]        $options Array of options passed directly to HttpDownloader constructor
      * @return HttpDownloader
      */
-    public static function createHttpDownloader(IOInterface $io, Config $config, $options = array())
+    public static function createHttpDownloader(IOInterface $io, Config $config, $options = array()): HttpDownloader
     {
         static $warned = false;
         $disableTls = false;
@@ -693,9 +663,6 @@ class Factory
         return $httpDownloader;
     }
 
-    /**
-     * @return bool
-     */
     private static function useXdg(): bool
     {
         foreach (array_keys($_SERVER) as $key) {
@@ -713,7 +680,6 @@ class Factory
 
     /**
      * @throws \RuntimeException
-     * @return string
      */
     private static function getUserDir(): string
     {
diff --git a/src/Composer/Installer/LibraryInstaller.php b/src/Composer/Installer/LibraryInstaller.php
index 7490e11a5..8fff84d46 100644
--- a/src/Composer/Installer/LibraryInstaller.php
+++ b/src/Composer/Installer/LibraryInstaller.php
@@ -14,6 +14,7 @@ namespace Composer\Installer;
 
 use Composer\Composer;
 use Composer\IO\IOInterface;
+use Composer\PartialComposer;
 use Composer\Pcre\Preg;
 use Composer\Repository\InstalledRepositoryInterface;
 use Composer\Package\PackageInterface;
@@ -31,11 +32,11 @@ use Composer\Downloader\DownloadManager;
  */
 class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
 {
-    /** @var Composer */
+    /** @var PartialComposer */
     protected $composer;
     /** @var string */
     protected $vendorDir;
-    /** @var DownloadManager */
+    /** @var DownloadManager|null */
     protected $downloadManager;
     /** @var IOInterface */
     protected $io;
@@ -50,15 +51,15 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
      * Initializes library installer.
      *
      * @param IOInterface     $io
-     * @param Composer        $composer
+     * @param PartialComposer $composer
      * @param string|null     $type
      * @param Filesystem      $filesystem
      * @param BinaryInstaller $binaryInstaller
      */
-    public function __construct(IOInterface $io, Composer $composer, $type = 'library', Filesystem $filesystem = null, BinaryInstaller $binaryInstaller = null)
+    public function __construct(IOInterface $io, PartialComposer $composer, $type = 'library', Filesystem $filesystem = null, BinaryInstaller $binaryInstaller = null)
     {
         $this->composer = $composer;
-        $this->downloadManager = $composer->getDownloadManager();
+        $this->downloadManager = $composer instanceof Composer ? $composer->getDownloadManager() : null;
         $this->io = $io;
         $this->type = $type;
 
@@ -101,7 +102,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $this->initializeVendorDir();
         $downloadPath = $this->getInstallPath($package);
 
-        return $this->downloadManager->download($package, $downloadPath, $prevPackage);
+        return $this->getDownloadManager()->download($package, $downloadPath, $prevPackage);
     }
 
     /**
@@ -112,7 +113,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $this->initializeVendorDir();
         $downloadPath = $this->getInstallPath($package);
 
-        return $this->downloadManager->prepare($type, $package, $downloadPath, $prevPackage);
+        return $this->getDownloadManager()->prepare($type, $package, $downloadPath, $prevPackage);
     }
 
     /**
@@ -123,7 +124,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $this->initializeVendorDir();
         $downloadPath = $this->getInstallPath($package);
 
-        return $this->downloadManager->cleanup($type, $package, $downloadPath, $prevPackage);
+        return $this->getDownloadManager()->cleanup($type, $package, $downloadPath, $prevPackage);
     }
 
     /**
@@ -266,7 +267,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
     {
         $downloadPath = $this->getInstallPath($package);
 
-        return $this->downloadManager->install($package, $downloadPath);
+        return $this->getDownloadManager()->install($package, $downloadPath);
     }
 
     /**
@@ -295,7 +296,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
             $this->filesystem->rename($initialDownloadPath, $targetDownloadPath);
         }
 
-        return $this->downloadManager->update($initial, $target, $targetDownloadPath);
+        return $this->getDownloadManager()->update($initial, $target, $targetDownloadPath);
     }
 
     /**
@@ -305,7 +306,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
     {
         $downloadPath = $this->getPackageBasePath($package);
 
-        return $this->downloadManager->remove($package, $downloadPath);
+        return $this->getDownloadManager()->remove($package, $downloadPath);
     }
 
     /**
@@ -316,4 +317,11 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $this->filesystem->ensureDirectoryExists($this->vendorDir);
         $this->vendorDir = realpath($this->vendorDir);
     }
+
+    protected function getDownloadManager(): DownloadManager
+    {
+        assert($this->downloadManager instanceof DownloadManager, new \LogicException(self::class.' should be initialized with a fully loaded Composer instance to be able to install/... packages'));
+
+        return $this->downloadManager;
+    }
 }
diff --git a/src/Composer/Installer/PluginInstaller.php b/src/Composer/Installer/PluginInstaller.php
index e4d0f0404..ce4c75933 100644
--- a/src/Composer/Installer/PluginInstaller.php
+++ b/src/Composer/Installer/PluginInstaller.php
@@ -14,8 +14,10 @@ namespace Composer\Installer;
 
 use Composer\Composer;
 use Composer\IO\IOInterface;
+use Composer\PartialComposer;
 use Composer\Repository\InstalledRepositoryInterface;
 use Composer\Package\PackageInterface;
+use Composer\Plugin\PluginManager;
 use Composer\Util\Filesystem;
 use Composer\Util\Platform;
 use React\Promise\PromiseInterface;
@@ -28,13 +30,7 @@ use React\Promise\PromiseInterface;
  */
 class PluginInstaller extends LibraryInstaller
 {
-    /**
-     * Initializes Plugin installer.
-     *
-     * @param IOInterface $io
-     * @param Composer    $composer
-     */
-    public function __construct(IOInterface $io, Composer $composer, Filesystem $fs = null, BinaryInstaller $binaryInstaller = null)
+    public function __construct(IOInterface $io, PartialComposer $composer, Filesystem $fs = null, BinaryInstaller $binaryInstaller = null)
     {
         parent::__construct($io, $composer, 'composer-plugin', $fs, $binaryInstaller);
     }
@@ -73,7 +69,7 @@ class PluginInstaller extends LibraryInstaller
         return $promise->then(function () use ($package, $repo) {
             try {
                 Platform::workaroundFilesystemIssues();
-                $this->composer->getPluginManager()->registerPackage($package, true);
+                $this->getPluginManager()->registerPackage($package, true);
             } catch (\Exception $e) {
                 $this->rollbackInstall($e, $repo, $package);
             }
@@ -93,8 +89,8 @@ class PluginInstaller extends LibraryInstaller
         return $promise->then(function () use ($initial, $target, $repo) {
             try {
                 Platform::workaroundFilesystemIssues();
-                $this->composer->getPluginManager()->deactivatePackage($initial);
-                $this->composer->getPluginManager()->registerPackage($target, true);
+                $this->getPluginManager()->deactivatePackage($initial);
+                $this->getPluginManager()->registerPackage($target, true);
             } catch (\Exception $e) {
                 $this->rollbackInstall($e, $repo, $target);
             }
@@ -103,7 +99,7 @@ class PluginInstaller extends LibraryInstaller
 
     public function uninstall(InstalledRepositoryInterface $repo, PackageInterface $package)
     {
-        $this->composer->getPluginManager()->uninstallPackage($package);
+        $this->getPluginManager()->uninstallPackage($package);
 
         return parent::uninstall($repo, $package);
     }
@@ -114,4 +110,12 @@ class PluginInstaller extends LibraryInstaller
         parent::uninstall($repo, $package);
         throw $e;
     }
+
+    protected function getPluginManager(): PluginManager
+    {
+        assert($this->composer instanceof Composer, new \LogicException(self::class.' should be initialized with a fully loaded Composer instance.'));
+        $pluginManager = $this->composer->getPluginManager();
+
+        return $pluginManager;
+    }
 }
diff --git a/src/Composer/PartialComposer.php b/src/Composer/PartialComposer.php
index fead62258..891175066 100644
--- a/src/Composer/PartialComposer.php
+++ b/src/Composer/PartialComposer.php
@@ -1,8 +1,151 @@
 <?php
 
+/*
+ * This file is part of Composer.
+ *
+ * (c) Nils Adermann <naderman@naderman.de>
+ *     Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Composer;
 
+use Composer\Package\RootPackageInterface;
+use Composer\Util\Loop;
+use Composer\Repository\RepositoryManager;
+use Composer\Installer\InstallationManager;
+use Composer\EventDispatcher\EventDispatcher;
+
+/**
+ * @author Jordi Boggiano <j.boggiano@seld.be>
+ */
 class PartialComposer
 {
+    /**
+     * @var RootPackageInterface
+     */
+    private $package;
 
+    /**
+     * @var Loop
+     */
+    private $loop;
+
+    /**
+     * @var Repository\RepositoryManager
+     */
+    private $repositoryManager;
+
+    /**
+     * @var Installer\InstallationManager
+     */
+    private $installationManager;
+
+    /**
+     * @var Config
+     */
+    private $config;
+
+    /**
+     * @var EventDispatcher
+     */
+    private $eventDispatcher;
+
+    /**
+     * @return void
+     */
+    public function setPackage(RootPackageInterface $package): void
+    {
+        $this->package = $package;
+    }
+
+    /**
+     * @return RootPackageInterface
+     */
+    public function getPackage(): RootPackageInterface
+    {
+        return $this->package;
+    }
+
+    /**
+     * @return void
+     */
+    public function setConfig(Config $config): void
+    {
+        $this->config = $config;
+    }
+
+    /**
+     * @return Config
+     */
+    public function getConfig(): Config
+    {
+        return $this->config;
+    }
+
+    /**
+     * @return void
+     */
+    public function setLoop(Loop $loop): void
+    {
+        $this->loop = $loop;
+    }
+
+    /**
+     * @return Loop
+     */
+    public function getLoop(): Loop
+    {
+        return $this->loop;
+    }
+
+    /**
+     * @return void
+     */
+    public function setRepositoryManager(RepositoryManager $manager): void
+    {
+        $this->repositoryManager = $manager;
+    }
+
+    /**
+     * @return RepositoryManager
+     */
+    public function getRepositoryManager(): RepositoryManager
+    {
+        return $this->repositoryManager;
+    }
+
+    /**
+     * @return void
+     */
+    public function setInstallationManager(InstallationManager $manager): void
+    {
+        $this->installationManager = $manager;
+    }
+
+    /**
+     * @return InstallationManager
+     */
+    public function getInstallationManager(): InstallationManager
+    {
+        return $this->installationManager;
+    }
+
+    /**
+     * @return void
+     */
+    public function setEventDispatcher(EventDispatcher $eventDispatcher): void
+    {
+        $this->eventDispatcher = $eventDispatcher;
+    }
+
+    /**
+     * @return EventDispatcher
+     */
+    public function getEventDispatcher(): EventDispatcher
+    {
+        return $this->eventDispatcher;
+    }
 }
diff --git a/src/Composer/Plugin/PluginManager.php b/src/Composer/Plugin/PluginManager.php
index 296fb06f4..b849de107 100644
--- a/src/Composer/Plugin/PluginManager.php
+++ b/src/Composer/Plugin/PluginManager.php
@@ -13,6 +13,7 @@
 namespace Composer\Plugin;
 
 use Composer\Composer;
+use Composer\Autoload\AutoloadGenerator;
 use Composer\EventDispatcher\EventSubscriberInterface;
 use Composer\Installer\InstallerInterface;
 use Composer\IO\IOInterface;
@@ -20,6 +21,7 @@ use Composer\Package\BasePackage;
 use Composer\Package\CompletePackage;
 use Composer\Package\Package;
 use Composer\Package\Version\VersionParser;
+use Composer\PartialComposer;
 use Composer\Pcre\Preg;
 use Composer\Repository\RepositoryInterface;
 use Composer\Repository\InstalledRepository;
@@ -42,7 +44,7 @@ class PluginManager
     protected $composer;
     /** @var IOInterface */
     protected $io;
-    /** @var ?Composer */
+    /** @var PartialComposer|null */
     protected $globalComposer;
     /** @var VersionParser */
     protected $versionParser;
@@ -67,15 +69,7 @@ class PluginManager
     /** @var int */
     private static $classCounter = 0;
 
-    /**
-     * Initializes plugin manager
-     *
-     * @param IOInterface $io
-     * @param Composer    $composer
-     * @param Composer    $globalComposer
-     * @param bool        $disablePlugins
-     */
-    public function __construct(IOInterface $io, Composer $composer, Composer $globalComposer = null, $disablePlugins = false)
+    public function __construct(IOInterface $io, Composer $composer, PartialComposer $globalComposer = null, bool $disablePlugins = false)
     {
         $this->io = $io;
         $this->composer = $composer;
@@ -118,10 +112,9 @@ class PluginManager
         }
 
         $repo = $this->composer->getRepositoryManager()->getLocalRepository();
-        $globalRepo = $this->globalComposer ? $this->globalComposer->getRepositoryManager()->getLocalRepository() : null;
         $this->deactivateRepository($repo, false);
-        if ($globalRepo) {
-            $this->deactivateRepository($globalRepo, true);
+        if ($this->globalComposer !== null) {
+            $this->deactivateRepository($this->globalComposer->getRepositoryManager()->getLocalRepository(), true);
         }
     }
 
@@ -137,10 +130,8 @@ class PluginManager
 
     /**
      * Gets global composer or null when main composer is not fully loaded
-     *
-     * @return Composer|null
      */
-    public function getGlobalComposer(): ?Composer
+    public function getGlobalComposer(): ?PartialComposer
     {
         return $this->globalComposer;
     }
@@ -558,6 +549,7 @@ class PluginManager
             return $this->composer->getInstallationManager()->getInstallPath($package);
         }
 
+        assert(null !== $this->globalComposer);
         return $this->globalComposer->getInstallationManager()->getInstallPath($package);
     }
 
diff --git a/tests/Composer/Test/Json/JsonValidationExceptionTest.php b/tests/Composer/Test/Json/JsonValidationExceptionTest.php
index 0d6feb050..7f959ae93 100644
--- a/tests/Composer/Test/Json/JsonValidationExceptionTest.php
+++ b/tests/Composer/Test/Json/JsonValidationExceptionTest.php
@@ -19,10 +19,10 @@ class JsonValidationExceptionTest extends TestCase
 {
     /**
      * @dataProvider errorProvider
-     * @param string|null $message
-     * @param string[]|null $errors
+     * @param string[] $errors
+     * @param string[] $expectedErrors
      */
-    public function testGetErrors($message, $errors, $expectedMessage, $expectedErrors): void
+    public function testGetErrors(?string $message, array $errors, string $expectedMessage, array $expectedErrors): void
     {
         $object = new JsonValidationException($message, $errors);
         $this->assertSame($expectedMessage, $object->getMessage());
diff --git a/tests/Composer/Test/Mock/FactoryMock.php b/tests/Composer/Test/Mock/FactoryMock.php
index 127720666..8db24fe11 100644
--- a/tests/Composer/Test/Mock/FactoryMock.php
+++ b/tests/Composer/Test/Mock/FactoryMock.php
@@ -17,6 +17,7 @@ use Composer\Package\Loader\RootPackageLoader;
 use Composer\Composer;
 use Composer\Config;
 use Composer\Factory;
+use Composer\PartialComposer;
 use Composer\Repository\RepositoryManager;
 use Composer\Package\Version\VersionGuesser;
 use Composer\Package\Version\VersionParser;
@@ -32,7 +33,7 @@ use Composer\Util\ProcessExecutor;
 
 class FactoryMock extends Factory
 {
-    public static function createConfig(IOInterface $io = null, $cwd = null): Config
+    public static function createConfig(IOInterface $io = null, ?string $cwd = null): Config
     {
         $config = new Config(true, $cwd);
 
@@ -59,7 +60,7 @@ class FactoryMock extends Factory
         return new InstallationManagerMock();
     }
 
-    protected function createDefaultInstallers(InstallationManager $im, Composer $composer, IOInterface $io, ProcessExecutor $process = null): void
+    protected function createDefaultInstallers(InstallationManager $im, PartialComposer $composer, IOInterface $io, ProcessExecutor $process = null): void
     {
     }