Bug 531061 - Asynchronous/non-blocking Java completion

Change-Id: I122f94304b3be477fd26df180cbdbcb0098508d8
Signed-off-by: Mickael Istria <mistria@redhat.com>

13 files changed, 412 insertions, 78 deletions

diff --git a/org.eclipse.jdt.text.tests/plugin.xml b/org.eclipse.jdt.text.tests/plugin.xml
index 8931f92480..49a78dccb8 100644
--- a/org.eclipse.jdt.text.tests/plugin.xml
+++ b/org.eclipse.jdt.text.tests/plugin.xml
@@ -36,5 +36,15 @@
             type="java.lang.Object">
       </propertyTester>
    </extension>
-
+	<extension
+		point="org.eclipse.jdt.ui.javaCompletionProposalComputer"
+		id="LongCompletionProposalComputer">
+		<javaCompletionProposalComputer
+			class="org.eclipse.jdt.text.tests.contentassist.LongCompletionProposalComputer"
+			categoryId="org.eclipse.jdt.ui.javaAllProposalCategory"
+			requiresUIThread="false">
+			<partition type="__dftl_partition_content_type"/>
+			<partition type="__java_string"/>
+		</javaCompletionProposalComputer>
+	</extension>
 </plugin>
diff --git a/org.eclipse.jdt.text.tests/src/org/eclipse/jdt/text/tests/contentassist/CheckUIThreadReactivityThread.java b/org.eclipse.jdt.text.tests/src/org/eclipse/jdt/text/tests/contentassist/CheckUIThreadReactivityThread.java
new file mode 100644
index 0000000000..cc14e27238
--- /dev/null
+++ b/org.eclipse.jdt.text.tests/src/org/eclipse/jdt/text/tests/contentassist/CheckUIThreadReactivityThread.java
@@ -0,0 +1,53 @@
+/*******************************************************************************
+ * Copyright (c) 2019 Red Hat Inc., and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+package org.eclipse.jdt.text.tests.contentassist;
+
+import org.eclipse.swt.widgets.Display;
+
+/**
+ * This thread polls the display every 50milliseconds with a null UI operation
+ * request and computes how long it take for display to process the request.
+ * The time to handle request is time when display is busy doing other work, so
+ * it's actually a UI Freeze.
+ * 
+ * This could be moved to SWT or other common place where we need to check
+ * UI Freezes and reused form there,
+ */
+public class CheckUIThreadReactivityThread extends Thread {
+
+	long pauseBetweenEachPing = 50;
+	final private Display fDisplay;
+
+	private long maxDuration;
+
+	public CheckUIThreadReactivityThread(Display display) {
+		fDisplay= display;
+	}
+
+	@Override
+	public void run() {
+		while (!isInterrupted()) {
+			long duration = System.currentTimeMillis();
+			fDisplay.syncExec(() -> {}); // do nothing, but in UI Thread
+			duration = System.currentTimeMillis() - duration;
+			maxDuration = Math.max(duration, maxDuration);
+			try {
+				sleep(pauseBetweenEachPing);
+			} catch (InterruptedException e) {
+				// nothing
+			}
+		}
+	}
+
+	public long getMaxDuration() {
+		return maxDuration;
+	}
+}
diff --git a/org.eclipse.jdt.text.tests/src/org/eclipse/jdt/text/tests/contentassist/ContentAssistAndThreadsTest.java b/org.eclipse.jdt.text.tests/src/org/eclipse/jdt/text/tests/contentassist/ContentAssistAndThreadsTest.java
new file mode 100644
index 0000000000..2fc1067f73
--- /dev/null
+++ b/org.eclipse.jdt.text.tests/src/org/eclipse/jdt/text/tests/contentassist/ContentAssistAndThreadsTest.java
@@ -0,0 +1,144 @@
+/*******************************************************************************
+ * Copyright (c) 2019 Red Hat Inc., and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+package org.eclipse.jdt.text.tests.contentassist;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.stream.Collectors;
+
+import org.junit.After;
+import org.junit.Test;
+
+import org.eclipse.jdt.testplugin.JavaProjectHelper;
+
+import org.eclipse.swt.widgets.Composite;
+import org.eclipse.swt.widgets.Display;
+import org.eclipse.swt.widgets.Shell;
+import org.eclipse.swt.widgets.Table;
+import org.eclipse.swt.widgets.TableItem;
+import org.eclipse.swt.widgets.Widget;
+
+import org.eclipse.core.runtime.IStatus;
+import org.eclipse.core.runtime.NullProgressMonitor;
+
+import org.eclipse.jface.text.contentassist.ContentAssistant;
+
+import org.eclipse.ui.texteditor.ContentAssistAction;
+import org.eclipse.ui.texteditor.ITextEditorActionConstants;
+
+import org.eclipse.jdt.core.ICompilationUnit;
+import org.eclipse.jdt.core.IJavaProject;
+import org.eclipse.jdt.core.IPackageFragment;
+import org.eclipse.jdt.core.IPackageFragmentRoot;
+
+import org.eclipse.jdt.ui.JavaUI;
+import org.eclipse.jdt.ui.PreferenceConstants;
+import org.eclipse.jdt.ui.text.IJavaPartitions;
+
+import org.eclipse.jdt.internal.ui.JavaPlugin;
+import org.eclipse.jdt.internal.ui.javaeditor.JavaEditor;
+import org.eclipse.jdt.internal.ui.text.java.JavaCompletionProcessor;
+
+public class ContentAssistAndThreadsTest extends AbstractCompletionTest {
+
+	@After
+	public void resetPreference() {
+		JavaPlugin.getDefault().getPreferenceStore().setToDefault(PreferenceConstants.CODEASSIST_NONUITHREAD_COMPUTATION);
+	}
+
+	@Test
+	public void testComputeCompletionInNonUIThread() throws Exception {
+		IJavaProject fJProject1= JavaProjectHelper.createJavaProject("TestProject1", "bin");
+		JavaProjectHelper.addRTJar(fJProject1);
+		IPackageFragmentRoot sourceFolder= JavaProjectHelper.addSourceContainer(fJProject1, "src");
+		IPackageFragment pack1= sourceFolder.createPackageFragment("test1", false, null);
+		ICompilationUnit cu= pack1.createCompilationUnit("Blah.java", "", true, new NullProgressMonitor());
+		JavaEditor part= (JavaEditor) JavaUI.openInEditor(cu);
+		ContentAssistant assistant= new ContentAssistant();
+		assistant.setDocumentPartitioning(IJavaPartitions.JAVA_PARTITIONING);
+		JavaCompletionProcessor javaProcessor= new JavaCompletionProcessor(part, assistant, getContentType());
+		AtomicReference<Throwable> exception = new AtomicReference<>();
+		List<IStatus> errors = new ArrayList<>();
+		JavaPlugin.getDefault().getLog().addLogListener((status, plugin) -> {
+			if (status.getSeverity() >= IStatus.WARNING) {
+				errors.add(status);
+			}
+		});
+		Thread thread = new Thread(() -> {
+			try {
+				javaProcessor.computeCompletionProposals(part.getViewer(), 0);
+				// a popup can be shown and block the thread in case of error
+			} catch (Exception e) {
+				exception.set(e);
+			}
+		});
+		thread.start();
+		thread.join();
+		if (exception.get() != null) {
+			exception.get().printStackTrace();
+		}
+		assertNull(exception.get());
+		assertEquals(Collections.emptyList(), errors);
+	}
+
+	@Test
+	public void testLongNonUIThreadContentAssistDoesntFreezeUI() throws Exception {
+		JavaPlugin.getDefault().getPreferenceStore().setValue(PreferenceConstants.CODEASSIST_NONUITHREAD_COMPUTATION, true);
+		IJavaProject fJProject1= JavaProjectHelper.createJavaProject("TestProject1", "bin");
+		JavaProjectHelper.addRTJar(fJProject1);
+		IPackageFragmentRoot sourceFolder= JavaProjectHelper.addSourceContainer(fJProject1, "src");
+		IPackageFragment pack1= sourceFolder.createPackageFragment("test1", false, null);
+		ICompilationUnit cu= pack1.createCompilationUnit("Blah.java", LongCompletionProposalComputer.CONTENT_TRIGGER_STRING, true, new NullProgressMonitor());
+		JavaEditor part= (JavaEditor) JavaUI.openInEditor(cu);
+		final Set<Shell> beforeShells = Arrays.stream(part.getSite().getShell().getDisplay().getShells()).filter(Shell::isVisible).collect(Collectors.toSet());
+		Display display= part.getViewer().getTextWidget().getDisplay();
+		ContentAssistAction action = (ContentAssistAction) part.getAction(ITextEditorActionConstants.CONTENT_ASSIST);
+		action.update();
+		CheckUIThreadReactivityThread thread = new CheckUIThreadReactivityThread(display);
+		thread.start();
+		display.asyncExec(() -> action.run()); // mustn't be synchronous or CheckUIThreadReactivityTest can miss it.
+		try {
+			assertTrue("Missing completion proposal", new org.eclipse.jdt.text.tests.performance.DisplayHelper() {
+				@Override
+				protected boolean condition() {
+					Set<Shell> newShells = Arrays.stream(part.getSite().getShell().getDisplay().getShells()).filter(Shell::isVisible).collect(Collectors.toSet());
+					newShells.removeAll(beforeShells);
+					if (!newShells.isEmpty()) {
+						Table completionTable = findCompletionSelectionControl(newShells.iterator().next());
+						return Arrays.stream(completionTable.getItems()).map(TableItem::getText).anyMatch(LongCompletionProposalComputer.CONTENT_TRIGGER_STRING::equals);
+					}
+					return false;
+				}
+			}.waitForCondition(display, 3000));
+		} finally {
+			thread.interrupt();
+		}
+		assertTrue("UI was frozen for " + thread.getMaxDuration(), thread.getMaxDuration() < 1000);
+	}
+
+	private Table findCompletionSelectionControl(Widget control) {
+		if (control instanceof Table) {
+			return (Table)control;
+		} else if (control instanceof Composite) {
+			for (Widget child : ((Composite)control).getChildren()) {
+				Table res = findCompletionSelectionControl(child);
+				if (res != null) {
+					return res;
+				}
+			}
+		}
+		return null;
+	}
+}
diff --git a/org.eclipse.jdt.text.tests/src/org/eclipse/jdt/text/tests/contentassist/LongCompletionProposalComputer.java b/org.eclipse.jdt.text.tests/src/org/eclipse/jdt/text/tests/contentassist/LongCompletionProposalComputer.java
new file mode 100644
index 0000000000..1dacabe983
--- /dev/null
+++ b/org.eclipse.jdt.text.tests/src/org/eclipse/jdt/text/tests/contentassist/LongCompletionProposalComputer.java
@@ -0,0 +1,62 @@
+/*******************************************************************************
+ * Copyright (c) 2019 Red Hat Inc., and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+package org.eclipse.jdt.text.tests.contentassist;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.eclipse.core.runtime.IProgressMonitor;
+
+import org.eclipse.jface.text.contentassist.CompletionProposal;
+import org.eclipse.jface.text.contentassist.ICompletionProposal;
+import org.eclipse.jface.text.contentassist.IContextInformation;
+
+import org.eclipse.jdt.ui.text.java.ContentAssistInvocationContext;
+import org.eclipse.jdt.ui.text.java.IJavaCompletionProposalComputer;
+
+import org.eclipse.jdt.internal.ui.JavaPlugin;
+
+public class LongCompletionProposalComputer implements IJavaCompletionProposalComputer {
+
+	public static final String CONTENT_TRIGGER_STRING = "longCompletion";
+
+	@Override
+	public void sessionStarted() {
+	}
+
+	@Override
+	public List<ICompletionProposal> computeCompletionProposals(ContentAssistInvocationContext context, IProgressMonitor monitor) {
+		if (context.getDocument().get().contains(CONTENT_TRIGGER_STRING)) {
+			try {
+				Thread.sleep(1000);
+			} catch (InterruptedException e) {
+				JavaPlugin.log(e);
+			}
+			return Collections.singletonList(new CompletionProposal(CONTENT_TRIGGER_STRING, 0, 0, 0, null, CONTENT_TRIGGER_STRING, null, null));
+		}
+		return Collections.emptyList();
+	}
+
+	@Override
+	public List<IContextInformation> computeContextInformation(ContentAssistInvocationContext context, IProgressMonitor monitor) {
+		return Collections.emptyList();
+	}
+
+	@Override
+	public String getErrorMessage() {
+		return null;
+	}
+
+	@Override
+	public void sessionEnded() {
+	}
+
+}
diff --git a/org.eclipse.jdt.ui/META-INF/MANIFEST.MF b/org.eclipse.jdt.ui/META-INF/MANIFEST.MF
index a20ac02cfb..80123f900f 100644
--- a/org.eclipse.jdt.ui/META-INF/MANIFEST.MF
+++ b/org.eclipse.jdt.ui/META-INF/MANIFEST.MF
@@ -3,7 +3,7 @@ Automatic-Module-Name: org.eclipse.jdt.ui
 Bundle-ManifestVersion: 2
 Bundle-Name: %pluginName
 Bundle-SymbolicName: org.eclipse.jdt.ui; singleton:=true
-Bundle-Version: 3.20.100.qualifier
+Bundle-Version: 3.21.0.qualifier
 Bundle-Activator: org.eclipse.jdt.internal.ui.JavaPlugin
 Bundle-ActivationPolicy: lazy
 Bundle-Vendor: %providerName
diff --git a/org.eclipse.jdt.ui/pom.xml b/org.eclipse.jdt.ui/pom.xml
index 7d0bf4d14f..609886acbb 100644
--- a/org.eclipse.jdt.ui/pom.xml
+++ b/org.eclipse.jdt.ui/pom.xml
@@ -18,7 +18,7 @@
   </parent>
   <groupId>org.eclipse.jdt</groupId>
   <artifactId>org.eclipse.jdt.ui</artifactId>
-  <version>3.20.100-SNAPSHOT</version>
+  <version>3.21.0-SNAPSHOT</version>
   <packaging>eclipse-plugin</packaging>
   <properties>
     <code.ignoredWarnings>-warn:-deprecation,unavoidableGenericProblems</code.ignoredWarnings>
diff --git a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/CodeAssistAdvancedConfigurationBlock.java b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/CodeAssistAdvancedConfigurationBlock.java
index 8f753a99fa..8909196293 100644
--- a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/CodeAssistAdvancedConfigurationBlock.java
+++ b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/CodeAssistAdvancedConfigurationBlock.java
@@ -20,6 +20,7 @@ import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.stream.Collectors;
 
 import org.eclipse.swt.SWT;
 import org.eclipse.swt.events.SelectionAdapter;
@@ -53,9 +54,7 @@ import org.eclipse.jface.layout.PixelConverter;
 import org.eclipse.jface.resource.ImageDescriptor;
 import org.eclipse.jface.resource.JFaceResources;
 import org.eclipse.jface.viewers.ArrayContentProvider;
-import org.eclipse.jface.viewers.CheckStateChangedEvent;
 import org.eclipse.jface.viewers.CheckboxTableViewer;
-import org.eclipse.jface.viewers.ICheckStateListener;
 import org.eclipse.jface.viewers.IStructuredSelection;
 import org.eclipse.jface.viewers.ITableLabelProvider;
 import org.eclipse.jface.viewers.LabelProvider;
@@ -92,12 +91,14 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 	private static final Key PREF_EXCLUDED_CATEGORIES= getJDTUIKey(PreferenceConstants.CODEASSIST_EXCLUDED_CATEGORIES);
 	private static final Key PREF_CATEGORY_ORDER= getJDTUIKey(PreferenceConstants.CODEASSIST_CATEGORY_ORDER);
 	private static final Key PREF_CODEASSIST_TIMEOUT_FOR_PARAMETER_NAME_FROM_ATTACHED_JAVADOC= getJDTCoreKey(JavaCore.TIMEOUT_FOR_PARAMETER_NAME_FROM_ATTACHED_JAVADOC);
+	private static final Key PREF_CODEASSIST_NONUITHREAD_COMPUTATION = getJDTUIKey(PreferenceConstants.CODEASSIST_NONUITHREAD_COMPUTATION);
 
 	private static Key[] getAllKeys() {
 		return new Key[] {
 				PREF_EXCLUDED_CATEGORIES,
 				PREF_CATEGORY_ORDER,
-				PREF_CODEASSIST_TIMEOUT_FOR_PARAMETER_NAME_FROM_ATTACHED_JAVADOC
+				PREF_CODEASSIST_TIMEOUT_FOR_PARAMETER_NAME_FROM_ATTACHED_JAVADOC,
+				PREF_CODEASSIST_NONUITHREAD_COMPUTATION
 		};
 	}
 
@@ -108,8 +109,9 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		 */
 		@Override
 		public Image getColumnImage(Object element, int columnIndex) {
-			if (columnIndex == 0)
+			if (columnIndex == 0) {
 				return ((ModelElement) element).getImage();
+			}
 			return null;
 		}
 
@@ -145,8 +147,9 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		 */
 		@Override
 		public Image getColumnImage(Object element, int columnIndex) {
-			if (columnIndex == 0)
+			if (columnIndex == 0) {
 				return ((ModelElement) element).getImage();
+			}
 			return null;
 		}
 
@@ -220,8 +223,9 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
     		StringBuilder buf= new StringBuilder();
 			for (ModelElement item : fElements) {
 				boolean included= changed == item ? isInDefaultCategory : item.isInDefaultCategory();
-				if (!included)
+				if (!included) {
 					buf.append(item.getId() + SEPARATOR);
+				}
 			}
 
     		String newValue= buf.toString();
@@ -257,8 +261,9 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
     	private int readOrderPreference(CompletionProposalCategory cat) {
     		for (String sortOrderId : getTokens(getValue(PREF_CATEGORY_ORDER), SEPARATOR)) {
 				String[] idAndRank= getTokens(sortOrderId, COLON);
-				if (idAndRank[0].equals(cat.getId()))
+				if (idAndRank[0].equals(cat.getId())) {
 					return Integer.parseInt(idAndRank[1]);
+				}
 			}
 			return LIMIT - 1;
     	}
@@ -304,16 +309,18 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 			return fPreferenceModel.readInclusionPreference(fCategory);
 		}
 		void setInDefaultCategory(boolean included) {
-			if (included != isInDefaultCategory())
+			if (included != isInDefaultCategory()) {
 				fPreferenceModel.writeInclusionPreference(this, included);
+			}
 		}
 		String getId() {
 			return fCategory.getId();
 		}
 		int getRank() {
 			int rank= getInternalRank();
-			if (rank > PreferenceModel.LIMIT)
+			if (rank > PreferenceModel.LIMIT) {
 				return rank - PreferenceModel.LIMIT;
+			}
 			return rank;
 		}
 		void moveUp() {
@@ -330,8 +337,9 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		}
 
 		void setSeparateCommand(boolean separate) {
-			if (separate != isSeparateCommand())
+			if (separate != isSeparateCommand()) {
 				fPreferenceModel.writeOrderPreference(this, separate);
+			}
 		}
 
 		void update() {
@@ -387,6 +395,8 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
         createFiller(composite, columns);
 
 		createParameterTimeoutControl(composite, columns);
+		createFiller(composite, columns);
+		createNonUIThreadControl(composite, columns);
 
 		updateControls();
 		if (fModel.elements.size() > 0) {
@@ -400,13 +410,30 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		return scrolled;
 	}
 
+	private void createNonUIThreadControl(Composite composite, int columns) {
+		PixelConverter pixelConverter= new PixelConverter(composite);
+		String str= PreferencesMessages.CodeAssistAdvancedConfigurationBlock_nonUIThread;
+		Button checkbox = addCheckBox(composite, str, PREF_CODEASSIST_NONUITHREAD_COMPUTATION, new String[] { Boolean.TRUE.toString(), Boolean.FALSE.toString() }, pixelConverter.convertWidthInCharsToPixels(7));
+		checkbox.setLayoutData(new GridData(GridData.FILL, GridData.FILL, true, false, columns, 1));
+		CompletionProposalComputerRegistry registry= CompletionProposalComputerRegistry.getDefault();
+		if (registry.computingCompletionRequiresUIThread()) {
+			Label label = new Label(composite, SWT.NONE);
+			label.setText(Messages.format(PreferencesMessages.CodeAssistAdvancedConfigurationBlock_nonUIThread_computersRequiringUIThread, registry.getComputersRequiringUIThreadNames().map(name -> "* " + name).collect(Collectors.joining("\n")))); //$NON-NLS-1$ //$NON-NLS-2$
+			GridData layoutData= new GridData(GridData.FILL, GridData.FILL, true, false, columns, 1);
+			layoutData.horizontalIndent = 30;
+			label.setLayoutData(layoutData);
+			label.setEnabled(false);
+		}
+	}
+
 	private void createDefaultLabel(Composite composite, int h_span) {
 	    final ICommandService commandSvc= PlatformUI.getWorkbench().getAdapter(ICommandService.class);
 		final Command command= commandSvc.getCommand(ITextEditorActionDefinitionIds.CONTENT_ASSIST_PROPOSALS);
 		ParameterizedCommand pCmd= new ParameterizedCommand(command, null);
 		String key= getKeyboardShortcut(pCmd);
-		if (key == null)
+		if (key == null) {
 			key= PreferencesMessages.CodeAssistAdvancedConfigurationBlock_no_shortcut;
+		}
 
 		PixelConverter pixelConverter= new PixelConverter(composite);
 		int width= pixelConverter.convertWidthInCharsToPixels(40);
@@ -440,13 +467,10 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		keyColumn.setText(PreferencesMessages.CodeAssistAdvancedConfigurationBlock_default_table_keybinding_column_title);
 		keyColumn.setResizable(false);
 
-		fDefaultViewer.addCheckStateListener(new ICheckStateListener() {
-			@Override
-			public void checkStateChanged(CheckStateChangedEvent event) {
-				boolean checked= event.getChecked();
-				ModelElement element= (ModelElement) event.getElement();
-				element.setInDefaultCategory(checked);
-			}
+		fDefaultViewer.addCheckStateListener(event -> {
+			boolean checked= event.getChecked();
+			ModelElement element= (ModelElement) event.getElement();
+			element.setInDefaultCategory(checked);
 		});
 
 		fDefaultViewer.setContentProvider(ArrayContentProvider.getInstance());
@@ -460,9 +484,9 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		final int HEADER_MARGIN= 20;
 		int minNameWidth= computeWidth(table, nameColumn.getText()) + HEADER_MARGIN;
 		int minKeyWidth= computeWidth(table, keyColumn.getText()) + HEADER_MARGIN;
-		for (int i= 0; i < fModel.elements.size(); i++) {
-			minNameWidth= Math.max(minNameWidth, computeWidth(table, labelProvider.getColumnText(fModel.elements.get(i), 0)) + ICON_AND_CHECKBOX_WITH);
-			minKeyWidth= Math.max(minKeyWidth, computeWidth(table, labelProvider.getColumnText(fModel.elements.get(i), 1)));
+		for (ModelElement element : fModel.elements) {
+			minNameWidth= Math.max(minNameWidth, computeWidth(table, labelProvider.getColumnText(element, 0)) + ICON_AND_CHECKBOX_WITH);
+			minKeyWidth= Math.max(minKeyWidth, computeWidth(table, labelProvider.getColumnText(element, 1)));
 		}
 
 		nameColumn.setWidth(minNameWidth);
@@ -530,19 +554,16 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		final int ICON_AND_CHECKBOX_WITH= 50;
 		final int HEADER_MARGIN= 20;
 		int minNameWidth= computeWidth(table, nameColumn.getText()) + HEADER_MARGIN;
-		for (int i= 0; i < fModel.elements.size(); i++) {
-			minNameWidth= Math.max(minNameWidth, computeWidth(table, labelProvider.getColumnText(fModel.elements.get(i), 0)) + ICON_AND_CHECKBOX_WITH);
+		for (ModelElement element : fModel.elements) {
+			minNameWidth= Math.max(minNameWidth, computeWidth(table, labelProvider.getColumnText(element, 0)) + ICON_AND_CHECKBOX_WITH);
 		}
 
 		nameColumn.setWidth(minNameWidth);
 
-		fSeparateViewer.addCheckStateListener(new ICheckStateListener() {
-			@Override
-			public void checkStateChanged(CheckStateChangedEvent event) {
-				boolean checked= event.getChecked();
-				ModelElement element= (ModelElement) event.getElement();
-				element.setSeparateCommand(checked);
-			}
+		fSeparateViewer.addCheckStateListener(event -> {
+			boolean checked= event.getChecked();
+			ModelElement element= (ModelElement) event.getElement();
+			element.setSeparateCommand(checked);
 		});
 
 		table.addSelectionListener(new SelectionAdapter() {
@@ -651,10 +672,12 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		List<ModelElement> separateChecked= new ArrayList<>(size);
 
 		for (ModelElement element : fModel.elements) {
-			if (element.isInDefaultCategory())
+			if (element.isInDefaultCategory()) {
 				defaultChecked.add(element);
-			if (element.isSeparateCommand())
+			}
+			if (element.isSeparateCommand()) {
 				separateChecked.add(element);
+			}
 		}
 
 		fDefaultViewer.setCheckedElements(defaultChecked.toArray(new Object[defaultChecked.size()]));
@@ -680,9 +703,9 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 	protected void validateSettings(Key changedKey, String oldValue, String newValue) {
 		if (changedKey == PREF_CODEASSIST_TIMEOUT_FOR_PARAMETER_NAME_FROM_ATTACHED_JAVADOC) {
 			final StatusInfo status= new StatusInfo();
-			if (newValue.length() == 0)
+			if (newValue.length() == 0) {
 				status.setError(PreferencesMessages.CodeAssistAdvancedConfigurationBlock_parameterNameFromAttachedJavadoc_timeout_emptyInput);
-			else {
+			} else {
 				try {
 					int number= Integer.parseInt(newValue);
 					int min= 0;
@@ -716,8 +739,7 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 	 */
 	@Override
 	public void dispose() {
-		for (Iterator<Image> it= fImages.values().iterator(); it.hasNext();) {
-			Image image= it.next();
+		for (Image image : fImages.values()) {
 			image.dispose();
 		}
 
@@ -725,8 +747,9 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 	}
 
 	private int computeWidth(Control control, String name) {
-		if (name == null)
+		if (name == null) {
 			return 0;
+		}
 		GC gc= new GC(control);
 		try {
 			gc.setFont(JFaceResources.getDialogFont());
@@ -760,21 +783,24 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		fgLocalBindingManager.setBindings(bindingService.getBindings());
 		try {
 			Scheme activeScheme= bindingService.getActiveScheme();
-			if (activeScheme != null)
+			if (activeScheme != null) {
 				fgLocalBindingManager.setActiveScheme(activeScheme);
+			}
 		} catch (NotDefinedException e) {
 			JavaPlugin.log(e);
 		}
 
 		TriggerSequence[] bindings= fgLocalBindingManager.getActiveBindingsDisregardingContextFor(command);
-		if (bindings.length > 0)
+		if (bindings.length > 0) {
 			return bindings[0].format();
+		}
 		return null;
 	}
 
 	private Image getImage(ImageDescriptor imgDesc) {
-		if (imgDesc == null)
+		if (imgDesc == null) {
 			return null;
+		}
 
 		Image img= fImages.get(imgDesc);
 		if (img == null) {
diff --git a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/PreferencesMessages.java b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/PreferencesMessages.java
index 20ea4a6a84..2dc67c88d5 100644
--- a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/PreferencesMessages.java
+++ b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/PreferencesMessages.java
@@ -65,6 +65,8 @@ public final class PreferencesMessages extends NLS {
 	public static String CodeAssistAdvancedConfigurationBlock_parameterNameFromAttachedJavadoc_timeout_emptyInput;
 	public static String CodeAssistAdvancedConfigurationBlock_parameterNameFromAttachedJavadoc_timeout_invalidInput;
 	public static String CodeAssistAdvancedConfigurationBlock_parameterNameFromAttachedJavadoc_timeout_invalidRange;
+	public static String CodeAssistAdvancedConfigurationBlock_nonUIThread;
+	public static String CodeAssistAdvancedConfigurationBlock_nonUIThread_computersRequiringUIThread;
 	public static String ImportOrganizePreferencePage_title;
 	public static String ImportOrganizeConfigurationBlock_order_label;
 	public static String ImportOrganizeConfigurationBlock_other_static;
diff --git a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/PreferencesMessages.properties b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/PreferencesMessages.properties
index 65522c1769..261b5953c8 100644
--- a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/PreferencesMessages.properties
+++ b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/preferences/PreferencesMessages.properties
@@ -950,6 +950,9 @@ CodeAssistAdvancedConfigurationBlock_separate_table_category_column_title=Separa
 CodeAssistAdvancedConfigurationBlock_Up=&Up
 CodeAssistAdvancedConfigurationBlock_Down=D&own
 CodeAssistAdvancedConfigurationBlock_parameterNameFromAttachedJavadoc_timeout=&Timeout for fetching a parameter name from attached Javadoc (ms):
+CodeAssistAdvancedConfigurationBlock_nonUIThread=Do not block &UI Thread while computing completion proposals (does not affect open editors)
+CodeAssistAdvancedConfigurationBlock_nonUIThread_computersRequiringUIThread=\u26A0\uFE0F This setting will be ignored because the following contributions require UI Thread:\n\
+{0}
 
 CodeAssistAdvancedConfigurationBlock_parameterNameFromAttachedJavadoc_timeout_emptyInput=Empty input.
 CodeAssistAdvancedConfigurationBlock_parameterNameFromAttachedJavadoc_timeout_invalidInput=''{0}'' is not a valid input.
diff --git a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/text/java/CompletionProposalComputerRegistry.java b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/text/java/CompletionProposalComputerRegistry.java
index 33dd482a47..a892d355b2 100644
--- a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/text/java/CompletionProposalComputerRegistry.java
+++ b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/text/java/CompletionProposalComputerRegistry.java
@@ -24,6 +24,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.StringTokenizer;
+import java.util.stream.Stream;
 
 import org.eclipse.swt.SWT;
 import org.eclipse.swt.events.SelectionAdapter;
@@ -526,4 +527,8 @@ public final class CompletionProposalComputerRegistry {
 	public boolean computingCompletionRequiresUIThread() {
 		return fDescriptors.stream().anyMatch(CompletionProposalComputerDescriptor::requiresUIThread);
 	}
+
+	public Stream<String> getComputersRequiringUIThreadNames() {
+		return fDescriptors.stream().filter(CompletionProposalComputerDescriptor::requiresUIThread).map(CompletionProposalComputerDescriptor::getName);
+	}
 }
diff --git a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/text/java/ContentAssistProcessor.java b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/text/java/ContentAssistProcessor.java
index c07448d1ae..7199d2f09f 100644
--- a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/text/java/ContentAssistProcessor.java
+++ b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/text/java/ContentAssistProcessor.java
@@ -32,6 +32,7 @@ import org.eclipse.swt.layout.GridLayout;
 import org.eclipse.swt.widgets.Button;
 import org.eclipse.swt.widgets.Composite;
 import org.eclipse.swt.widgets.Control;
+import org.eclipse.swt.widgets.Display;
 import org.eclipse.swt.widgets.Link;
 import org.eclipse.swt.widgets.Shell;
 
@@ -110,8 +111,9 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 		 */
 		@Override
 		public void assistSessionStarted(ContentAssistEvent event) {
-			if (event.processor != ContentAssistProcessor.this)
+			if (event.processor != ContentAssistProcessor.this) {
 				return;
+			}
 
 			fIterationGesture= getIterationGesture();
 			KeySequence binding= getIterationBinding();
@@ -145,7 +147,7 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 
 		/**
 		 * Returns the categories that need to be notified when a session starts and ends.
-		 * 
+		 *
 		 * @return the current categories
 		 * @since 3.8.1
 		 */
@@ -155,14 +157,16 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 			// Currently enabled categories for this session
 			if (fCategoryIteration != null) {
 				Iterator<List<CompletionProposalCategory>> it= fCategoryIteration.iterator();
-				while (it.hasNext())
+				while (it.hasNext()) {
 					currentCategories.addAll(it.next());
+				}
 			}
 
 			// Backwards compatibility: notify all categories which have no enablement expression
 			for (CompletionProposalCategory cat : fCategories) {
-				if (cat.getEnablementExpression() == null)
+				if (cat.getEnablementExpression() == null) {
 					currentCategories.add(cat);
+				}
 			}
 
 			return currentCategories;
@@ -173,8 +177,9 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 		 */
 		@Override
 		public void assistSessionEnded(ContentAssistEvent event) {
-			if (event.processor != ContentAssistProcessor.this)
+			if (event.processor != ContentAssistProcessor.this) {
 				return;
+			}
 
 			for (CompletionProposalCategory cat : getCategoriesToNotify()) {
 				cat.sessionEnded();
@@ -222,14 +227,7 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 	 */
 	private static final String PREF_WARN_ABOUT_EMPTY_ASSIST_CATEGORY= "EmptyDefaultAssistCategory"; //$NON-NLS-1$
 
-	private static final Comparator<CompletionProposalCategory> ORDER_COMPARATOR= new Comparator<CompletionProposalCategory>() {
-
-		@Override
-		public int compare(CompletionProposalCategory d1, CompletionProposalCategory d2) {
-			return d1.getSortOrder() - d2.getSortOrder();
-		}
-
-	};
+	private static final Comparator<CompletionProposalCategory> ORDER_COMPARATOR= (d1, d2) -> d1.getSortOrder() - d2.getSortOrder();
 
 	private final List<CompletionProposalCategory> fCategories;
 	private final String fPartition;
@@ -256,7 +254,7 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 	 * Flag indicating whether any completion engine associated with this processor requests
 	 * resorting of its proposals after filtering is triggered. Filtering is, e.g., triggered when a
 	 * user continues typing with an open completion window.
-	 * 
+	 *
 	 * @since 3.8
 	 */
 	private boolean fNeedsSortingAfterFiltering;
@@ -292,10 +290,11 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 		long collect= JavaPlugin.DEBUG_RESULT_COLLECTOR ? System.currentTimeMillis() : 0;
 
 		monitor.subTask(JavaTextMessages.ContentAssistProcessor_sorting_proposals);
-		if (fNeedsSortingAfterFiltering)
+		if (fNeedsSortingAfterFiltering) {
 			setContentAssistSorter();
-		else
+		} else {
 			proposals= sortProposals(proposals, monitor, context);
+		}
 		fNumberOfComputedResults= proposals.size();
 		long filter= JavaPlugin.DEBUG_RESULT_COLLECTOR ? System.currentTimeMillis() : 0;
 
@@ -334,11 +333,13 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 			List<ICompletionProposal> computed= cat.computeCompletionProposals(context, fPartition, new SubProgressMonitor(monitor, 1));
 			proposals.addAll(computed);
 			needsSortingAfterFiltering= needsSortingAfterFiltering || (cat.isSortingAfterFilteringNeeded() && !computed.isEmpty());
-			if (fErrorMessage == null)
+			if (fErrorMessage == null) {
 				fErrorMessage= cat.getErrorMessage();
+			}
 		}
-		if (fNeedsSortingAfterFiltering && !needsSortingAfterFiltering)
+		if (fNeedsSortingAfterFiltering && !needsSortingAfterFiltering) {
 			fAssistant.setSorter(null);
+		}
 		fNeedsSortingAfterFiltering= needsSortingAfterFiltering;
 		return proposals;
 	}
@@ -388,8 +389,9 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 		for (CompletionProposalCategory cat : providers) {
 			List<IContextInformation> computed= cat.computeContextInformation(context, fPartition, new SubProgressMonitor(monitor, 1));
 			proposals.addAll(computed);
-			if (fErrorMessage == null)
+			if (fErrorMessage == null) {
 				fErrorMessage= cat.getErrorMessage();
+			}
 		}
 
 		return proposals;
@@ -442,10 +444,12 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 	 */
 	@Override
 	public String getErrorMessage() {
-		if (fErrorMessage != null)
+		if (fErrorMessage != null) {
 			return fErrorMessage;
-		if (fNumberOfComputedResults > 0)
+		}
+		if (fNumberOfComputedResults > 0) {
 			return null;
+		}
 		return JavaUIMessages.JavaEditor_codeassist_noCompletions;
 	}
 
@@ -483,11 +487,17 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 	}
 
 	private List<CompletionProposalCategory> getCategories() {
-		if (fCategoryIteration == null)
+		if (fCategoryIteration == null) {
 			return fCategories;
+		}
 
 		int iteration= fRepetition % fCategoryIteration.size();
-		fAssistant.setStatusMessage(createIterationMessage());
+		String iterationMessage = createIterationMessage();
+		if (Display.getCurrent() != null) {
+			fAssistant.setStatusMessage(iterationMessage);
+		} else {
+			Display.getDefault().asyncExec(() -> fAssistant.setStatusMessage(iterationMessage));
+		}
 		fAssistant.setEmptyMessage(createEmptyMessage());
 		fRepetition++;
 
@@ -515,9 +525,10 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 		List<CompletionProposalCategory> included= getDefaultCategoriesUnchecked();
 
 		if (fComputerRegistry.hasUninstalledComputers(fPartition, included)) {
-			if (informUserAboutEmptyDefaultCategory())
+			if (informUserAboutEmptyDefaultCategory()) {
 				// preferences were restored - recompute the default categories
 				included= getDefaultCategoriesUnchecked();
+			}
 			fComputerRegistry.resetUnistalledComputers();
 		}
 
@@ -527,15 +538,16 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 	private List<CompletionProposalCategory> getDefaultCategoriesUnchecked() {
 		List<CompletionProposalCategory> included= new ArrayList<>();
 		for (CompletionProposalCategory category : fCategories) {
-			if (checkDefaultEnablement(category)) 
+			if (checkDefaultEnablement(category)) {
 				included.add(category);
+			}
 		}
 		return included;
 	}
 
 	/**
 	 * Determine whether the category is enabled by default.
-	 * 
+	 *
 	 * @param category the category to check
 	 * @return <code>true</code> if this category is enabled by default, <code>false</code>
 	 *         otherwise
@@ -620,8 +632,9 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 	        		store.setToDefault(PreferenceConstants.CODEASSIST_CATEGORY_ORDER);
 	        		store.setToDefault(PreferenceConstants.CODEASSIST_EXCLUDED_CATEGORIES);
 	        	}
-	        	if (settingsId == returnValue)
+				if (settingsId == returnValue) {
 					PreferencesUtil.createPreferenceDialogOn(shell, "org.eclipse.jdt.ui.preferences.CodeAssistPreferenceAdvanced", null, null).open(); //$NON-NLS-1$
+				}
 	        	fComputerRegistry.reload();
 	        	return true;
 	        }
@@ -632,16 +645,17 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 	private List<CompletionProposalCategory> getSeparateCategories() {
 		ArrayList<CompletionProposalCategory> sorted= new ArrayList<>();
 		for (CompletionProposalCategory category : fCategories) {
-			if (checkSeparateEnablement(category))
+			if (checkSeparateEnablement(category)) {
 				sorted.add(category);
+			}
 		}
 		Collections.sort(sorted, ORDER_COMPARATOR);
 		return sorted;
 	}
-	
+
 	/**
 	 * Determine whether the category is enabled for separate use.
-	 * 
+	 *
 	 * @param category the category to check
 	 * @return <code>true</code> if this category is enabled for separate use, <code>false</code>
 	 *         otherwise
@@ -661,8 +675,9 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 
 	private String getCategoryLabel(int repetition) {
 		int iteration= repetition % fCategoryIteration.size();
-		if (iteration == 0)
+		if (iteration == 0) {
 			return JavaTextMessages.ContentAssistProcessor_defaultProposalCategory;
+		}
 		return toString(fCategoryIteration.get(iteration).get(0));
 	}
 
@@ -680,14 +695,15 @@ public class ContentAssistProcessor implements IContentAssistProcessor {
 	private KeySequence getIterationBinding() {
 	    final IBindingService bindingSvc= PlatformUI.getWorkbench().getAdapter(IBindingService.class);
 		TriggerSequence binding= bindingSvc.getBestActiveBindingFor(ITextEditorActionDefinitionIds.CONTENT_ASSIST_PROPOSALS);
-		if (binding instanceof KeySequence)
+		if (binding instanceof KeySequence) {
 			return (KeySequence) binding;
+		}
 		return null;
     }
 
 	/**
 	 * Sets the current proposal sorter into the content assistant.
-	 * 
+	 *
 	 * @since 3.8
 	 * @see ProposalSorterRegistry#getCurrentSorter() the sorter used if <code>true</code>
 	 */
diff --git a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/PreferenceConstants.java b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/PreferenceConstants.java
index c786b78460..5b0857b5a3 100644
--- a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/PreferenceConstants.java
+++ b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/PreferenceConstants.java
@@ -2956,6 +2956,18 @@ public class PreferenceConstants {
 	public final static String FORMATTER_COMMENT_FORMATHTML= "comment_format_html"; //$NON-NLS-1$
 
 	/**
+	 * A named preference that controls whether completion processors should be called from the non-UI thread,
+	 * when they declare ability to work from non-UI Thread.
+	 * <p>Completion processors can declare whether they
+	 * require UI Thread or not in their extension description, see {@link org.eclipse.jdt.internal.ui.text.java.CompletionProposalComputerDescriptor#requiresUIThread()}.</p>
+	 * <p>Value is of type <code>Boolean</code></p>
+	 *
+	 * @since 3.21
+	 */
+	public static final String CODEASSIST_NONUITHREAD_COMPUTATION= "content_assist_noUIThread_computation"; //$NON-NLS-1$
+
+
+	/**
 	 * A named preference that controls if the Java code assist gets auto activated.
 	 * <p>
 	 * Value is of type <code>Boolean</code>.
@@ -4087,6 +4099,7 @@ public class PreferenceConstants {
 		store.setDefault(PreferenceConstants.CODEASSIST_AUTOACTIVATION_DELAY, 0);
 		store.setDefault(PreferenceConstants.CODEASSIST_AUTOINSERT, true);
 		store.setDefault(PreferenceConstants.CODEASSIST_DISABLE_COMPLETION_PROPOSAL_TRIGGER_CHARS, false);
+		store.setDefault(PreferenceConstants.CODEASSIST_NONUITHREAD_COMPUTATION, false);
 		store.setDefault(PreferenceConstants.PREF_MIN_CHAIN_LENGTH, 2);
 		store.setDefault(PreferenceConstants.PREF_MAX_CHAIN_LENGTH, 4);
 		store.setDefault(PreferenceConstants.PREF_MAX_CHAINS, 20);
diff --git a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/text/JavaSourceViewerConfiguration.java b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/text/JavaSourceViewerConfiguration.java
index 20ef33bec7..a6154dd513 100644
--- a/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/text/JavaSourceViewerConfiguration.java
+++ b/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/text/JavaSourceViewerConfiguration.java
@@ -86,6 +86,7 @@ import org.eclipse.jdt.internal.ui.text.JavaReconciler;
 import org.eclipse.jdt.internal.ui.text.PreferencesAdapter;
 import org.eclipse.jdt.internal.ui.text.SingleTokenJavaScanner;
 import org.eclipse.jdt.internal.ui.text.correction.JavaCorrectionAssistant;
+import org.eclipse.jdt.internal.ui.text.java.CompletionProposalComputerRegistry;
 import org.eclipse.jdt.internal.ui.text.java.ContentAssistProcessor;
 import org.eclipse.jdt.internal.ui.text.java.JavaAutoIndentStrategy;
 import org.eclipse.jdt.internal.ui.text.java.JavaCodeScanner;
@@ -434,8 +435,7 @@ public class JavaSourceViewerConfiguration extends TextSourceViewerConfiguration
 	public IContentAssistant getContentAssistant(ISourceViewer sourceViewer) {
 
 		if (getEditor() != null) {
-
-			ContentAssistant assistant= new ContentAssistant();
+			ContentAssistant assistant= new ContentAssistant(fPreferenceStore.getBoolean(PreferenceConstants.CODEASSIST_NONUITHREAD_COMPUTATION) && !CompletionProposalComputerRegistry.getDefault().computingCompletionRequiresUIThread());
 			assistant.setDocumentPartitioning(getConfiguredDocumentPartitioning(sourceViewer));
 
 			assistant.setRestoreCompletionProposalSize(getSettings("completion_proposal_size")); //$NON-NLS-1$