Address PR review comments

- serverStatus.jsp: wrap dynamic substitutions in <c:out> so exception
  messages and monitor stats can't smuggle HTML. Closes the rmiError
  XSS path; the rest is defense in depth.
- DatabaseQueueMonitor: snapshot queueLevel/queueSize once so the
  message, stats, CloudWatch metric, and threshold check all see the
  same observation.
- SystemCpuMonitor: render "N/A" instead of a negative percentage when
  JMX can't sample the CPU load.
- PredictabilityMonitor: drop duplicate ArrayList/Collection/List
  imports left over from a 2015 merge.

Author: aaronbrethorst

transitclock/src/main/java/org/transitclock/monitoring/DatabaseQueueMonitor.java

@@ -73,30 +73,36 @@ protected boolean triggered() {
 			return false;
 
 		DataDbLogger dbLogger = core.getDbLogger();
-		
+
+		// Snapshot once: queueLevel/queueSize change under load and we want
+		// the message, stats, CloudWatch metric, and threshold check to
+		// agree on a single observation.
+		double queueLevel = dbLogger.queueLevel();
+		int queueSize = dbLogger.queueSize();
+
 		setMessage("Database queue fraction="
-				+ StringUtils.twoDigitFormat(dbLogger.queueLevel())
+				+ StringUtils.twoDigitFormat(queueLevel)
 				+ " while max allowed fraction="
 				+ StringUtils.twoDigitFormat(maxQueueFraction.getValue())
-				+ ", and items in queue=" + dbLogger.queueSize()
+				+ ", and items in queue=" + queueSize
 				+ ".",
-				dbLogger.queueLevel());
+				queueLevel);
 
-		addStat("Queue level", StringUtils.percentFormat(dbLogger.queueLevel()));
+		addStat("Queue level", StringUtils.percentFormat(queueLevel));
 		addStat("Max allowed", StringUtils.percentFormat(maxQueueFraction.getValue()));
-		addStat("Items queued", String.valueOf(dbLogger.queueSize()));
+		addStat("Items queued", String.valueOf(queueSize));
+
+        cloudwatchService.saveMetric("PredictionDatabaseQueuePercentageLevel", queueLevel, 1, CloudwatchService.MetricType.AVERAGE, CloudwatchService.ReportingIntervalTimeUnit.MINUTE, false);
 
-        cloudwatchService.saveMetric("PredictionDatabaseQueuePercentageLevel", dbLogger.queueLevel(), 1, CloudwatchService.MetricType.AVERAGE, CloudwatchService.ReportingIntervalTimeUnit.MINUTE, false);
-		
 		// Determine the threshold for triggering. If already triggered
 		// then lower the threshold by maxQueueFractionGap in order
 		// to prevent lots of e-mail being sent out if the value is
 		// dithering around maxQueueFraction.
 		double threshold = maxQueueFraction.getValue();
 		if (wasTriggered())
 			threshold -= maxQueueFractionGap.getValue();
-		
-		return dbLogger.queueLevel() > threshold; 
+
+		return queueLevel > threshold;
 	}
 
 	/* (non-Javadoc)

transitclock/src/main/java/org/transitclock/monitoring/PredictabilityMonitor.java

@@ -29,10 +29,6 @@
 import org.transitclock.utils.EmailSender;
 import org.transitclock.utils.StringUtils;
 
-import java.util.ArrayList;
-import java.util.Collection;
-import java.util.List;
-
 /**
  * Monitors how many vehicles are predictable compared to how many active blocks
  * there currently are.

transitclock/src/main/java/org/transitclock/monitoring/SystemCpuMonitor.java

@@ -121,7 +121,8 @@ protected boolean triggered() {
 					+ ".",
 					cpuLoad);
 
-			addStat("CPU load", StringUtils.percentFormat(cpuLoad));
+			// JMX returns a negative value when CPU load cannot be sampled.
+			addStat("CPU load", cpuLoad >= 0.0 ? StringUtils.percentFormat(cpuLoad) : "N/A");
 			addStat("Limit", StringUtils.percentFormat(cpuThreshold.getValue()));
 
 			// Determine the threshold for triggering. If already triggered

transitclockWebapp/src/main/webapp/status/serverStatus.jsp

@@ -57,7 +57,7 @@ try {
   <c:when test="${not empty rmiError}">
     <div class="rounded-lg border border-red-200 bg-red-50 p-4">
       <h2 class="text-sm font-semibold text-red-900"><fmt:message key="div.coreUnreachable" /></h2>
-      <p class="mt-1 text-sm text-red-800 font-mono break-all">${rmiError}</p>
+      <p class="mt-1 text-sm text-red-800 font-mono break-all"><c:out value="${rmiError}"/></p>
     </div>
   </c:when>
   <c:otherwise>
@@ -66,22 +66,22 @@ try {
         <c:if test="${not empty monitorResult.message}">
           <article class="bg-white border border-gray-200 rounded-lg p-4">
             <h2 class="text-xs font-semibold uppercase tracking-wider text-gray-500">
-              ${monitorResult.type}
+              <c:out value="${monitorResult.type}"/>
             </h2>
             <c:choose>
               <c:when test="${not empty monitorResult.stats}">
                 <dl class="mt-3 grid grid-cols-2 gap-x-4 gap-y-1.5 text-sm">
                   <c:forEach var="stat" items="${monitorResult.stats}">
                     <div class="flex items-baseline gap-1.5 col-span-2 sm:col-span-1">
-                      <dt class="text-gray-500">${stat.key}:</dt>
-                      <dd class="font-mono text-gray-900">${stat.value}</dd>
+                      <dt class="text-gray-500"><c:out value="${stat.key}"/>:</dt>
+                      <dd class="font-mono text-gray-900"><c:out value="${stat.value}"/></dd>
                     </div>
                   </c:forEach>
                 </dl>
               </c:when>
               <c:otherwise>
                 <p class="mt-2 text-sm text-gray-900 leading-relaxed tabular-nums">
-                  ${monitorResult.message}
+                  <c:out value="${monitorResult.message}"/>
                 </p>
               </c:otherwise>
             </c:choose>