diff --git a/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/BaseExternalTool.java b/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/BaseExternalTool.java
index 4a76a6f95765..d4fbf23aa6c4 100644
--- a/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/BaseExternalTool.java
+++ b/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/BaseExternalTool.java
@@ -46,7 +46,7 @@ abstract class BaseExternalTool implements DiffTool {
     myToolProperty = toolProperty;
   }
 
-  public final boolean canShow(DiffRequest request) {
+  public final boolean canShow(@NotNull DiffRequest request) {
     if (!isEnabled() || StringUtil.isEmpty(getToolPath())) return false;
     return isAvailable(request);
   }
@@ -56,10 +56,10 @@ abstract class BaseExternalTool implements DiffTool {
     return null;
   }
 
-  public abstract boolean isAvailable(DiffRequest request);
+  public abstract boolean isAvailable(@NotNull DiffRequest request);
 
   @Nullable
-  protected ContentExternalizer externalize(final DiffRequest request, final int index) {
+  protected ContentExternalizer externalize(@NotNull DiffRequest request, final int index) {
     final VirtualFile file = getLocalFile(request.getContents()[index].getFile());
 
     if (LocalFileExternalizer.canExternalizeAsFile(file)) {
@@ -81,7 +81,8 @@ abstract class BaseExternalTool implements DiffTool {
     return myEnableProperty.value(getProperties());
   }
 
-  protected List<String> getParameters(DiffRequest request) throws Exception {
+  @NotNull
+  protected List<String> getParameters(@NotNull DiffRequest request) throws Exception {
     final String p1 = convertToPath(request, 0);
     final String p2 = convertToPath(request, 1);
     final List<String> params = new ArrayList<String>();
diff --git a/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtCompareFiles.java b/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtCompareFiles.java
index 0594ab09c0c5..6f661acf9463 100644
--- a/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtCompareFiles.java
+++ b/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtCompareFiles.java
@@ -19,6 +19,7 @@ import com.intellij.openapi.diff.DiffContent;
 import com.intellij.openapi.diff.DiffRequest;
 import com.intellij.openapi.diff.impl.DiffUtil;
 import com.intellij.openapi.vfs.VirtualFile;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * @author Konstantin Bulenkov
@@ -30,7 +31,7 @@ class ExtCompareFiles extends BaseExternalTool {
     super(DiffManagerImpl.ENABLE_FILES, DiffManagerImpl.FILES_TOOL);
   }
 
-  public boolean isAvailable(DiffRequest request) {
+  public boolean isAvailable(@NotNull DiffRequest request) {
     final DiffContent[] contents = request.getContents();
     for (DiffContent content : contents) {
       final VirtualFile file = getLocalFile(content.getFile());
diff --git a/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtCompareFolders.java b/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtCompareFolders.java
index 14c0a9607293..6260176d3e26 100644
--- a/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtCompareFolders.java
+++ b/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtCompareFolders.java
@@ -18,6 +18,7 @@ package com.intellij.openapi.diff.impl.external;
 import com.intellij.openapi.diff.DiffContent;
 import com.intellij.openapi.diff.DiffRequest;
 import com.intellij.openapi.vfs.VirtualFile;
+import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 /**
@@ -31,7 +32,7 @@ class ExtCompareFolders extends BaseExternalTool {
   }
 
   @Override
-  public boolean isAvailable(DiffRequest request) {
+  public boolean isAvailable(@NotNull DiffRequest request) {
     final DiffContent[] contents = request.getContents();
     if (contents.length != 2) return false;
     if (externalize(request, 0) == null) return false;
@@ -40,7 +41,7 @@ class ExtCompareFolders extends BaseExternalTool {
   }
 
   @Nullable
-  protected ContentExternalizer externalize(DiffRequest request, int index) {
+  protected ContentExternalizer externalize(@NotNull DiffRequest request, int index) {
     final VirtualFile file = request.getContents()[index].getFile();
 
     if (!isLocalDirectory(file)) {
diff --git a/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtMergeFiles.java b/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtMergeFiles.java
index b65454d64206..6d80a99e5b69 100644
--- a/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtMergeFiles.java
+++ b/platform/platform-impl/src/com/intellij/openapi/diff/impl/external/ExtMergeFiles.java
@@ -25,6 +25,7 @@ import com.intellij.openapi.diff.impl.mergeTool.MergeRequestImpl;
 import com.intellij.openapi.ui.DialogWrapper;
 import com.intellij.openapi.ui.Messages;
 import com.intellij.openapi.util.text.StringUtil;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -40,7 +41,7 @@ public class ExtMergeFiles extends BaseExternalTool {
   }
 
   @Override
-  public boolean isAvailable(DiffRequest request) {
+  public boolean isAvailable(@NotNull DiffRequest request) {
     if (!(request instanceof MergeRequestImpl)) return false;
     DiffContent[] contents = request.getContents();
     if (contents.length != 3) return false;
@@ -52,7 +53,8 @@ public class ExtMergeFiles extends BaseExternalTool {
   }
 
   @Override
-  protected List<String> getParameters(DiffRequest request) throws Exception {
+  @NotNull
+  protected List<String> getParameters(@NotNull DiffRequest request) throws Exception {
     final List<String> params = new ArrayList<String>();
     String result = ((MergeRequestImpl)request).getResultContent().getFile().getPath();
     String left = externalize(request, 0).getContentFile().getPath();
@@ -68,7 +70,8 @@ public class ExtMergeFiles extends BaseExternalTool {
     return params;
   }
 
-  public void show(DiffRequest request) {
+  @Override
+  public void show(@NotNull DiffRequest request) {
     saveContents(request);
 
     int result = DialogWrapper.CANCEL_EXIT_CODE;