diff --git a/java/java-tests/testSrc/com/intellij/psi/StubAstSwitchTest.groovy b/java/java-tests/testSrc/com/intellij/psi/StubAstSwitchTest.groovy
index a996ce26aecf..3ad83a0f32bd 100644
--- a/java/java-tests/testSrc/com/intellij/psi/StubAstSwitchTest.groovy
+++ b/java/java-tests/testSrc/com/intellij/psi/StubAstSwitchTest.groovy
@@ -17,7 +17,9 @@ package com.intellij.psi
 
 import com.intellij.openapi.application.ApplicationManager
 import com.intellij.psi.impl.source.PsiFileImpl
+import com.intellij.psi.util.PsiTreeUtil
 import com.intellij.testFramework.fixtures.LightCodeInsightFixtureTestCase
+import com.intellij.reference.SoftReference
 
 /**
  * @author peter
@@ -30,13 +32,29 @@ class StubAstSwitchTest extends LightCodeInsightFixtureTestCase {
     def cls = ((PsiJavaFile)file).classes[0]
     assert file.stub
 
+    def oldCount = psiManager.modificationTracker.javaStructureModificationCount
+
     ApplicationManager.application.runWriteAction { file.virtualFile.setBinaryContent(file.virtualFile.contentsToByteArray()) }
     assert file.stub
 
-    //todo
-    //either: assert !cls.valid
-    //or: assert cls == PsiTreeUtil.findElementOfClassAtOffset(file, 1, PsiClass, false)
-    //assert !file.stub
+    assert psiManager.modificationTracker.javaStructureModificationCount != oldCount
 
+    assert !cls.valid
+    assert cls != PsiTreeUtil.findElementOfClassAtOffset(file, 1, PsiClass, false)
+    assert !file.stub
+  }
+
+  public void "test reachable psi classes remain valid when nothing changes"() {
+    int count = 1000
+    List<SoftReference<PsiClass>> classList = (0..<count).collect { new SoftReference(myFixture.addClass("class Foo$it {}")) }
+    System.gc()
+    System.gc()
+    System.gc()
+    assert classList.every {
+      def cls = it.get()
+      if (!cls || cls.valid) return true
+      cls.text //load AST
+      return cls.valid
+    }
   }
 }
diff --git a/platform/core-impl/src/com/intellij/extapi/psi/StubBasedPsiElementBase.java b/platform/core-impl/src/com/intellij/extapi/psi/StubBasedPsiElementBase.java
index 83dd5c3e9abd..588957ca7594 100644
--- a/platform/core-impl/src/com/intellij/extapi/psi/StubBasedPsiElementBase.java
+++ b/platform/core-impl/src/com/intellij/extapi/psi/StubBasedPsiElementBase.java
@@ -154,11 +154,11 @@ public class StubBasedPsiElementBase<T extends StubElement> extends ASTDelegateP
   public boolean isValid() {
     T stub = myStub;
     if (stub != null) {
-      if (stub instanceof PsiFileStub) {
-        return stub.getPsi().isValid();
+      PsiElement parentPsi = stub.getParentStub().getPsi();
+      if (parentPsi instanceof PsiFileImpl) {
+        return ((PsiFileImpl) parentPsi).isStubBasedChildValid(this);
       }
-
-      return stub.getParentStub().getPsi().isValid();
+      return parentPsi.isValid();
     }
 
     return super.isValid();
diff --git a/platform/core-impl/src/com/intellij/psi/impl/source/PsiFileImpl.java b/platform/core-impl/src/com/intellij/psi/impl/source/PsiFileImpl.java
index dd202dd57883..d3ff3e9ae771 100644
--- a/platform/core-impl/src/com/intellij/psi/impl/source/PsiFileImpl.java
+++ b/platform/core-impl/src/com/intellij/psi/impl/source/PsiFileImpl.java
@@ -249,6 +249,20 @@ public abstract class PsiFileImpl extends ElementBase implements PsiFileEx, PsiF
     return findTreeForStub(ast, stubs, stub);
   }
 
+  public boolean isStubBasedChildValid(@NotNull StubBasedPsiElementBase psi) {
+    StubElement fileStub = getStub();
+    if (fileStub != null) {
+      synchronized (myStubLock) {
+        fileStub = getStub();
+        StubElement childStub = psi.getStub();
+        if (fileStub != null && childStub != null && fileStub != childStub.getParentStub()) {
+          return false;
+        }
+      }
+    }
+    return isValid();
+  }
+
   @Nullable
   private static ASTNode findTreeForStub(ASTNode tree, final Iterator<StubElement<?>> stubs, final StubElement stub) {
     final IElementType type = tree.getElementType();