diff --git a/tools/core/src/main/java/org/teavm/tooling/sources/DirectorySourceFileProvider.java b/tools/core/src/main/java/org/teavm/tooling/sources/DirectorySourceFileProvider.java
index aaf0e2abc..1d19ce942 100644
--- a/tools/core/src/main/java/org/teavm/tooling/sources/DirectorySourceFileProvider.java
+++ b/tools/core/src/main/java/org/teavm/tooling/sources/DirectorySourceFileProvider.java
@@ -20,10 +20,6 @@ import java.io.FileInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 
-/**
- *
- * @author Alexey Andreev
- */
 public class DirectorySourceFileProvider implements SourceFileProvider {
     private File baseDirectory;
 
@@ -40,8 +36,26 @@ public class DirectorySourceFileProvider implements SourceFileProvider {
     }
 
     @Override
-    public InputStream openSourceFile(String fullPath) throws IOException {
+    public SourceFileInfo getSourceFile(String fullPath) throws IOException {
         File file = new File(baseDirectory, fullPath);
-        return file.exists() ? new FileInputStream(file) : null;
+        return file.exists() ? new DirectorySourceFile(file) : null;
+    }
+
+    static class DirectorySourceFile implements SourceFileInfo {
+        private File file;
+
+        DirectorySourceFile(File file) {
+            this.file = file;
+        }
+
+        @Override
+        public long lastModified() {
+            return file.lastModified();
+        }
+
+        @Override
+        public InputStream open() throws IOException {
+            return new FileInputStream(file);
+        }
     }
 }
diff --git a/tools/core/src/main/java/org/teavm/tooling/sources/JarSourceFileProvider.java b/tools/core/src/main/java/org/teavm/tooling/sources/JarSourceFileProvider.java
index 07269d76f..7d474f10e 100644
--- a/tools/core/src/main/java/org/teavm/tooling/sources/JarSourceFileProvider.java
+++ b/tools/core/src/main/java/org/teavm/tooling/sources/JarSourceFileProvider.java
@@ -24,10 +24,6 @@ import java.util.Set;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipFile;
 
-/**
- *
- * @author Alexey Andreev
- */
 public class JarSourceFileProvider implements SourceFileProvider {
     private File file;
     private ZipFile zipFile;
@@ -58,11 +54,31 @@ public class JarSourceFileProvider implements SourceFileProvider {
     }
 
     @Override
-    public InputStream openSourceFile(String fullPath) throws IOException {
+    public SourceFileInfo getSourceFile(String fullPath) throws IOException {
         if (zipFile == null || !sourceFiles.contains(fullPath)) {
             return null;
         }
         ZipEntry entry = zipFile.getEntry(fullPath);
-        return entry != null ? zipFile.getInputStream(entry) : null;
+        return entry != null ? new JarSourceFile(zipFile, entry) : null;
+    }
+
+    static class JarSourceFile implements SourceFileInfo {
+        private ZipFile file;
+        private ZipEntry entry;
+
+        JarSourceFile(ZipFile file, ZipEntry entry) {
+            this.file = file;
+            this.entry = entry;
+        }
+
+        @Override
+        public long lastModified() {
+            return entry.getTime();
+        }
+
+        @Override
+        public InputStream open() throws IOException {
+            return file.getInputStream(entry);
+        }
     }
 }
diff --git a/tools/core/src/main/java/org/teavm/tooling/sources/SourceFileInfo.java b/tools/core/src/main/java/org/teavm/tooling/sources/SourceFileInfo.java
new file mode 100644
index 000000000..e71c453b5
--- /dev/null
+++ b/tools/core/src/main/java/org/teavm/tooling/sources/SourceFileInfo.java
@@ -0,0 +1,25 @@
+/*
+ *  Copyright 2017 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.tooling.sources;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+public interface SourceFileInfo {
+    long lastModified();
+
+    InputStream open() throws IOException;
+}
diff --git a/tools/core/src/main/java/org/teavm/tooling/sources/SourceFileProvider.java b/tools/core/src/main/java/org/teavm/tooling/sources/SourceFileProvider.java
index c1dbebd9b..278606430 100644
--- a/tools/core/src/main/java/org/teavm/tooling/sources/SourceFileProvider.java
+++ b/tools/core/src/main/java/org/teavm/tooling/sources/SourceFileProvider.java
@@ -16,16 +16,11 @@
 package org.teavm.tooling.sources;
 
 import java.io.IOException;
-import java.io.InputStream;
 
-/**
- *
- * @author Alexey Andreev
- */
 public interface SourceFileProvider {
     void open() throws IOException;
 
     void close() throws IOException;
 
-    InputStream openSourceFile(String fullPath) throws IOException;
+    SourceFileInfo getSourceFile(String fullPath) throws IOException;
 }
diff --git a/tools/core/src/main/java/org/teavm/tooling/sources/SourceFilesCopier.java b/tools/core/src/main/java/org/teavm/tooling/sources/SourceFilesCopier.java
index 746f38bdf..54255392b 100644
--- a/tools/core/src/main/java/org/teavm/tooling/sources/SourceFilesCopier.java
+++ b/tools/core/src/main/java/org/teavm/tooling/sources/SourceFilesCopier.java
@@ -26,10 +26,6 @@ import org.teavm.model.MethodReader;
 import org.teavm.tooling.EmptyTeaVMToolLog;
 import org.teavm.tooling.TeaVMToolLog;
 
-/**
- *
- * @author Alexey Andreev
- */
 public class SourceFilesCopier {
     private TeaVMToolLog log = new EmptyTeaVMToolLog();
     private List<SourceFileProvider> sourceFileProviders;
@@ -66,15 +62,22 @@ public class SourceFilesCopier {
         }
         targetDirectory.mkdirs();
         for (String fileName : sourceFiles) {
-            try (InputStream input = findSourceFile(fileName)) {
-                if (input != null) {
-                    File outputFile = new File(targetDirectory, fileName);
-                    outputFile.getParentFile().mkdirs();
-                    try (OutputStream output = new FileOutputStream(outputFile)) {
-                        IOUtils.copy(input, output);
+            try {
+                SourceFileInfo sourceFile = findSourceFile(fileName);
+                File outputFile = new File(targetDirectory, fileName);
+                if (outputFile.exists() && outputFile.lastModified() > sourceFile.lastModified()
+                        && sourceFile.lastModified() > 0) {
+                    continue;
+                }
+                try (InputStream input = sourceFile.open()) {
+                    if (input != null) {
+                        outputFile.getParentFile().mkdirs();
+                        try (OutputStream output = new FileOutputStream(outputFile)) {
+                            IOUtils.copy(input, output);
+                        }
+                    } else {
+                        log.info("Missing source file: " + fileName);
                     }
-                } else {
-                    log.info("Missing source file: " + fileName);
                 }
             } catch (IOException e) {
                 log.warning("Could not copy source file " + fileName, e);
@@ -89,11 +92,11 @@ public class SourceFilesCopier {
         }
     }
 
-    private InputStream findSourceFile(String fileName) throws IOException {
+    private SourceFileInfo findSourceFile(String fileName) throws IOException {
         for (SourceFileProvider provider : sourceFileProviders) {
-            InputStream input = provider.openSourceFile(fileName);
-            if (input != null) {
-                return input;
+            SourceFileInfo sourceFile = provider.getSourceFile(fileName);
+            if (sourceFile != null) {
+                return sourceFile;
             }
         }
         return null;