Use Collections.synchronizedMap, add comments

src/main/java/net/fortuna/ical4j/model/DateTime.java

@@ -39,6 +39,7 @@
 import java.text.DateFormat;
 import java.text.ParseException;
 import java.text.SimpleDateFormat;
+import java.util.Collections;
 import java.util.Map;
 import java.util.WeakHashMap;
 
@@ -519,17 +520,28 @@ public int hashCode() {
 		return super.hashCode();
 	}
 
+	/**
+	 * This cache class is a workaround for DateFormat not being threadsafe.
+	 * We maintain map from Thread to DateFormat instance so that the instances
+	 * are not shared between threads (effectively a ThreadLocal).
+	 * TODO: once the project targets Java 8+, the new date utilities are
+	 * thread-safe and we should remove this code.
+	 */
 	private static class DateFormatCache {
 
-		private final Map<Thread, DateFormat> threadMap = new WeakHashMap<Thread, DateFormat>();
+		/**
+		 * This map needs to keep weak references (to avoid memory leaks - see r1.37)
+		 * and be thread-safe (since it may be concurrently modified in get() below).
+		 */
+		private final Map<Thread, DateFormat> threadMap = Collections.synchronizedMap(new WeakHashMap<Thread, DateFormat>());
 
 		private final DateFormat templateFormat;
 
 		private DateFormatCache(DateFormat dateFormat) {
 			this.templateFormat = dateFormat;
 		}
 
-		public synchronized DateFormat get() {
+		public DateFormat get() {
 			DateFormat dateFormat = threadMap.get(Thread.currentThread());
 			if (dateFormat == null) {
 				dateFormat = (DateFormat) templateFormat.clone();