diff --git a/fpdfsdk/include/javascript/JS_Runtime.h b/fpdfsdk/include/javascript/JS_Runtime.h
index 67ce9c0..dd21e6d 100644
--- a/fpdfsdk/include/javascript/JS_Runtime.h
+++ b/fpdfsdk/include/javascript/JS_Runtime.h
@@ -10,7 +10,6 @@
 #include <set>
 #include <utility>
 
-#include "../../../third_party/base/nonstd_unique_ptr.h"
 #include "../../../core/include/fxcrt/fx_basic.h"
 #include "../jsapi/fxjs_v8.h"
 #include "IJavaScript.h"
@@ -55,7 +54,6 @@
   std::set<FieldEvent> m_FieldEventSet;
   v8::Isolate* m_isolate;
   bool m_isolateManaged;
-  nonstd::unique_ptr<FXJS_ArrayBufferAllocator> m_pArrayBufferAllocator;
   v8::Global<v8::Context> m_context;
 };
 
diff --git a/fpdfsdk/include/jsapi/fxjs_v8.h b/fpdfsdk/include/jsapi/fxjs_v8.h
index 60fcea6..1705b89 100644
--- a/fpdfsdk/include/jsapi/fxjs_v8.h
+++ b/fpdfsdk/include/jsapi/fxjs_v8.h
@@ -62,9 +62,14 @@
 using FXJS_DESTRUCTOR = void (*)(v8::Local<v8::Object> obj);
 
 // Call before making FXJS_PrepareIsolate call.
-void FXJS_Initialize(unsigned int embedderDataSlot);
+void FXJS_Initialize(unsigned int embedderDataSlot, v8::Isolate* pIsolate);
 void FXJS_Release();
 
+// Gets the global isolate set by FXJS_Initialize(), or makes a new one each
+// time if there is no such isolate. Returns true if a new isolate had to be
+// created.
+bool FXJS_GetIsolate(v8::Isolate** pResultIsolate);
+
 // Call before making FXJS_Define* calls. Resources allocated here are cleared
 // as part of FXJS_ReleaseRuntime().
 void FXJS_PrepareIsolate(v8::Isolate* pIsolate);
diff --git a/fpdfsdk/src/fpdfview.cpp b/fpdfsdk/src/fpdfview.cpp
index 3603af6..0bf6426 100644
--- a/fpdfsdk/src/fpdfview.cpp
+++ b/fpdfsdk/src/fpdfview.cpp
@@ -14,6 +14,7 @@
 #include "../include/fsdk_define.h"
 #include "../include/fsdk_mgr.h"
 #include "../include/fsdk_rendercontext.h"
+#include "../include/jsapi/fxjs_v8.h"
 
 CPDF_CustomAccess::CPDF_CustomAccess(FPDF_FILEACCESS* pFileAccess) {
   if (pFileAccess)
@@ -109,6 +110,10 @@
     pModuleMgr->LoadEmbeddedCNS1CMaps();
     pModuleMgr->LoadEmbeddedKorea1CMaps();
   }
+  if (cfg && cfg->version >= 2) {
+    FXJS_Initialize(cfg->m_v8EmbedderSlot,
+                    reinterpret_cast<v8::Isolate*>(cfg->m_pIsolate));
+  }
 }
 
 DLLEXPORT void STDCALL FPDF_DestroyLibrary() {
diff --git a/fpdfsdk/src/javascript/JS_Runtime.cpp b/fpdfsdk/src/javascript/JS_Runtime.cpp
index 1e1486d..5c463ce 100644
--- a/fpdfsdk/src/javascript/JS_Runtime.cpp
+++ b/fpdfsdk/src/javascript/JS_Runtime.cpp
@@ -35,22 +35,17 @@
       m_bBlocking(FALSE),
       m_isolate(NULL),
       m_isolateManaged(false) {
-  unsigned int embedderDataSlot = 0;
-  if (m_pApp->GetFormFillInfo()->m_pJsPlatform->version >= 2) {
-    m_isolate = reinterpret_cast<v8::Isolate*>(
-        m_pApp->GetFormFillInfo()->m_pJsPlatform->m_isolate);
-    embedderDataSlot = pApp->GetFormFillInfo()->m_pJsPlatform->m_v8EmbedderSlot;
+  IPDF_JSPLATFORM* pPlatform = m_pApp->GetFormFillInfo()->m_pJsPlatform;
+  if (pPlatform->version <= 2) {
+    unsigned int embedderDataSlot = 0;
+    v8::Isolate* pExternalIsolate = nullptr;
+    if (pPlatform->version == 2) {
+      pExternalIsolate = reinterpret_cast<v8::Isolate*>(pPlatform->m_isolate);
+      embedderDataSlot = pPlatform->m_v8EmbedderSlot;
+    }
+    FXJS_Initialize(embedderDataSlot, pExternalIsolate);
   }
-  if (!m_isolate) {
-    m_pArrayBufferAllocator.reset(new FXJS_ArrayBufferAllocator());
-
-    v8::Isolate::CreateParams params;
-    params.array_buffer_allocator = m_pArrayBufferAllocator.get();
-    m_isolate = v8::Isolate::New(params);
-    m_isolateManaged = true;
-  }
-
-  FXJS_Initialize(embedderDataSlot);
+  m_isolateManaged = FXJS_GetIsolate(&m_isolate);
   DefineJSObjects();
 
   CJS_Context* pContext = (CJS_Context*)NewContext();
diff --git a/fpdfsdk/src/jsapi/fxjs_v8.cpp b/fpdfsdk/src/jsapi/fxjs_v8.cpp
index 667132f..ff45a7a 100644
--- a/fpdfsdk/src/jsapi/fxjs_v8.cpp
+++ b/fpdfsdk/src/jsapi/fxjs_v8.cpp
@@ -16,14 +16,14 @@
 const wchar_t kFXJSValueNameNull[] = L"null";
 const wchar_t kFXJSValueNameUndefined[] = L"undefined";
 
-static unsigned int g_embedderDataSlot = 1u;
-
 // Keep this consistent with the values defined in gin/public/context_holder.h
 // (without actually requiring a dependency on gin itself for the standalone
 // embedders of PDFIum). The value we want to use is:
 //   kPerContextDataStartIndex + kEmbedderPDFium, which is 3.
 static const unsigned int kPerContextDataIndex = 3u;
-
+static unsigned int g_embedderDataSlot = 1u;
+static v8::Isolate* g_isolate = nullptr;
+static FXJS_ArrayBufferAllocator* g_arrayBufferAllocator = nullptr;
 static v8::Global<v8::ObjectTemplate>* g_DefaultGlobalObjectTemplate = nullptr;
 
 class CFXJS_PrivateData {
@@ -131,6 +131,33 @@
   free(data);
 }
 
+void FXJS_Initialize(unsigned int embedderDataSlot, v8::Isolate* pIsolate) {
+  g_embedderDataSlot = embedderDataSlot;
+  g_isolate = pIsolate;
+}
+
+void FXJS_Release() {
+  g_DefaultGlobalObjectTemplate = nullptr;
+  g_isolate = nullptr;
+
+  delete g_arrayBufferAllocator;
+  g_arrayBufferAllocator = nullptr;
+}
+
+bool FXJS_GetIsolate(v8::Isolate** pResultIsolate) {
+  if (g_isolate) {
+    *pResultIsolate = g_isolate;
+    return false;
+  }
+  // Provide backwards compatibility when no external isolate.
+  if (!g_arrayBufferAllocator)
+    g_arrayBufferAllocator = new FXJS_ArrayBufferAllocator();
+  v8::Isolate::CreateParams params;
+  params.array_buffer_allocator = g_arrayBufferAllocator;
+  *pResultIsolate = v8::Isolate::New(params);
+  return true;
+}
+
 // static
 void FXJS_PerIsolateData::SetUp(v8::Isolate* pIsolate) {
   if (!pIsolate->GetData(g_embedderDataSlot))
@@ -143,13 +170,6 @@
       pIsolate->GetData(g_embedderDataSlot));
 }
 
-void FXJS_Initialize(unsigned int embedderDataSlot) {
-  g_embedderDataSlot = embedderDataSlot;
-}
-
-void FXJS_Release() {
-}
-
 int FXJS_DefineObj(v8::Isolate* pIsolate,
                    const wchar_t* sObjName,
                    FXJSOBJTYPE eObjType,
diff --git a/fpdfsdk/src/jsapi/fxjs_v8_embeddertest.cpp b/fpdfsdk/src/jsapi/fxjs_v8_embeddertest.cpp
index 7726c40..1c66039 100644
--- a/fpdfsdk/src/jsapi/fxjs_v8_embeddertest.cpp
+++ b/fpdfsdk/src/jsapi/fxjs_v8_embeddertest.cpp
@@ -30,7 +30,7 @@
 
     v8::Isolate::Scope isolate_scope(m_pIsolate);
     v8::HandleScope handle_scope(m_pIsolate);
-    FXJS_Initialize(0);
+    FXJS_Initialize(0, nullptr);
     FXJS_PerIsolateData::SetUp(m_pIsolate);
     FXJS_InitializeRuntime(m_pIsolate, nullptr, nullptr, m_pPersistentContext);
   }
diff --git a/public/fpdf_formfill.h b/public/fpdf_formfill.h
index 128c7ec..baab18a 100644
--- a/public/fpdf_formfill.h
+++ b/public/fpdf_formfill.h
@@ -293,17 +293,12 @@
 
   /* Version 2. */
 
-  /**
-  *   pointer to the v8::Isolate to use, or NULL to force PDFium to create one.
-  **/
-  void* m_isolate;
+  void* m_isolate;               /* Unused in v3, retain for compatibility. */
+  unsigned int m_v8EmbedderSlot; /* Unused in v3, retain for compatibility. */
 
-  /**
-   *   The embedder data slot to use in the v8::Isolate to store PDFium's
-   *   per-isolate data. The value needs to be between 0 and
-   *   v8::Internals::kNumIsolateDataLots (exclusive).
-   */
-  unsigned int m_v8EmbedderSlot;
+  /* Version 3. */
+  /* Version 3 moves m_Isolate and m_v8EmbedderSlot to FPDF_LIBRARY_CONFIG. */
+
 } IPDF_JSPLATFORM;
 
 // Flags for Cursor type
diff --git a/public/fpdfview.h b/public/fpdfview.h
index 9d199a1..ea45c29 100644
--- a/public/fpdfview.h
+++ b/public/fpdfview.h
@@ -130,7 +130,7 @@
 
 // Process-wide options for initializing library.
 typedef struct FPDF_LIBRARY_CONFIG_ {
-  // Version number of the interface. Currently must be 1.
+  // Version number of the interface. Currently must be 2.
   int version;
 
   // Array of paths to scan in place of the defaults when using built-in
@@ -138,6 +138,17 @@
   // The Array may be NULL itself to use the default paths. May be ignored
   // entirely depending upon the platform.
   const char** m_pUserFontPaths;
+
+  // Version 2.
+
+  // pointer to the v8::Isolate to use, or NULL to force PDFium to create one.
+  void* m_pIsolate;
+
+  // The embedder data slot to use in the v8::Isolate to store PDFium's
+  // per-isolate data. The value needs to be between 0 and
+  // v8::Internals::kNumIsolateDataLots (exclusive). Note that 0 is fine
+  // for most embedders.
+  unsigned int m_v8EmbedderSlot;
 } FPDF_LIBRARY_CONFIG;
 
 // Function: FPDF_InitLibraryWithConfig
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 1d8a77a..74e797b 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -437,7 +437,7 @@
 
   IPDF_JSPLATFORM platform_callbacks;
   memset(&platform_callbacks, '\0', sizeof(platform_callbacks));
-  platform_callbacks.version = 2;
+  platform_callbacks.version = 3;
   platform_callbacks.app_alert = ExampleAppAlert;
   platform_callbacks.Doc_gotoPage = ExampleDocGotoPage;
 
@@ -616,17 +616,19 @@
   v8::V8::SetSnapshotDataBlob(&snapshot);
 #endif  // V8_USE_EXTERNAL_STARTUP_DATA
 
-  if (options.font_directory.empty()) {
-    FPDF_InitLibrary();
-  } else {
-    const char* path_array[2];
+  FPDF_LIBRARY_CONFIG config;
+  config.version = 2;
+  config.m_pUserFontPaths = nullptr;
+  config.m_pIsolate = nullptr;
+  config.m_v8EmbedderSlot = 0;
+
+  const char* path_array[2];
+  if (!options.font_directory.empty()) {
     path_array[0] = options.font_directory.c_str();
     path_array[1] = nullptr;
-    FPDF_LIBRARY_CONFIG config;
-    config.version = 1;
     config.m_pUserFontPaths = path_array;
-    FPDF_InitLibraryWithConfig(&config);
   }
+  FPDF_InitLibraryWithConfig(&config);
 
   UNSUPPORT_INFO unsuppored_info;
   memset(&unsuppored_info, '\0', sizeof(unsuppored_info));