Use StringBuilder to construct a String instead of relying on appending where possible (#24753)

This PR revolves around places in the code where introducing a StringBuilder might make the construction
of a String easier to follow and also, maybe avoid a case where the compiler's very safe way of introducing 
StringBuilder instead of String might not always be optimal for performance.

client/rest/src/main/java/org/elasticsearch/client/RequestLogger.java

@@ -139,11 +139,12 @@ final class RequestLogger {
      * Creates curl output for given response
      */
     static String buildTraceResponse(HttpResponse httpResponse) throws IOException {
-        String responseLine = "# " + httpResponse.getStatusLine().toString();
+        StringBuilder responseLine = new StringBuilder();
+        responseLine.append("# ").append(httpResponse.getStatusLine());
         for (Header header : httpResponse.getAllHeaders()) {
-            responseLine += "\n# " + header.getName() + ": " + header.getValue();
+            responseLine.append("\n# ").append(header.getName()).append(": ").append(header.getValue());
         }
-        responseLine += "\n#";
+        responseLine.append("\n#");
         HttpEntity entity = httpResponse.getEntity();
         if (entity != null) {
             if (entity.isRepeatable() == false) {
@@ -158,11 +159,11 @@ final class RequestLogger {
             try (BufferedReader reader = new BufferedReader(new InputStreamReader(entity.getContent(), charset))) {
                 String line;
                 while( (line = reader.readLine()) != null) {
-                    responseLine += "\n# " + line;
+                    responseLine.append("\n# ").append(line);
                 }
             }
         }
-        return responseLine;
+        return responseLine.toString();
     }
 
     private static String getUri(RequestLine requestLine) {

core/src/main/java/org/elasticsearch/monitor/jvm/JvmInfo.java

@@ -299,19 +299,19 @@ public class JvmInfo implements Writeable, ToXContent {
     public int versionAsInteger() {
         try {
             int i = 0;
-            String sVersion = "";
+            StringBuilder sVersion = new StringBuilder();
             for (; i < version.length(); i++) {
                 if (!Character.isDigit(version.charAt(i)) && version.charAt(i) != '.') {
                     break;
                 }
                 if (version.charAt(i) != '.') {
-                    sVersion += version.charAt(i);
+                    sVersion.append(version.charAt(i));
                 }
             }
             if (i == 0) {
                 return -1;
             }
-            return Integer.parseInt(sVersion);
+            return Integer.parseInt(sVersion.toString());
         } catch (Exception e) {
             return -1;
         }
@@ -320,19 +320,19 @@ public class JvmInfo implements Writeable, ToXContent {
     public int versionUpdatePack() {
         try {
             int i = 0;
-            String sVersion = "";
+            StringBuilder sVersion = new StringBuilder();
             for (; i < version.length(); i++) {
                 if (!Character.isDigit(version.charAt(i)) && version.charAt(i) != '.') {
                     break;
                 }
                 if (version.charAt(i) != '.') {
-                    sVersion += version.charAt(i);
+                    sVersion.append(version.charAt(i));
                 }
             }
             if (i == 0) {
                 return -1;
             }
-            Integer.parseInt(sVersion);
+            Integer.parseInt(sVersion.toString());
             int from;
             if (version.charAt(i) == '_') {
                 // 1.7.0_4

core/src/main/java/org/elasticsearch/rest/BaseRestHandler.java

@@ -85,12 +85,12 @@ public abstract class BaseRestHandler extends AbstractComponent implements RestH
         final Set<String> invalids,
         final Set<String> candidates,
         final String detail) {
-        String message = String.format(
+        StringBuilder message = new StringBuilder(String.format(
             Locale.ROOT,
             "request [%s] contains unrecognized %s%s: ",
             request.path(),
             detail,
-            invalids.size() > 1 ? "s" : "");
+            invalids.size() > 1 ? "s" : ""));
         boolean first = true;
         for (final String invalid : invalids) {
             final LevensteinDistance ld = new LevensteinDistance();
@@ -108,17 +108,23 @@ public abstract class BaseRestHandler extends AbstractComponent implements RestH
                 else return a.v2().compareTo(b.v2());
             });
             if (first == false) {
-                message += ", ";
+                message.append(", ");
             }
-            message += "[" + invalid + "]";
+            message.append("[").append(invalid).append("]");
             final List<String> keys = scoredParams.stream().map(Tuple::v2).collect(Collectors.toList());
             if (keys.isEmpty() == false) {
-                message += " -> did you mean " + (keys.size() == 1 ? "[" + keys.get(0) + "]" : "any of " + keys.toString()) + "?";
+                message.append(" -> did you mean ");
+                if (keys.size() == 1) {
+                    message.append("[").append(keys.get(0)).append("]");
+                } else {
+                    message.append("any of ").append(keys.toString());
+                }
+                message.append("?");
             }
             first = false;
         }
 
-        return message;
+        return message.toString();
     }
 
     /**

test/framework/src/main/java/org/elasticsearch/ingest/RandomDocumentPicks.java

@@ -42,14 +42,14 @@ public final class RandomDocumentPicks {
      */
     public static String randomFieldName(Random random) {
         int numLevels = RandomNumbers.randomIntBetween(random, 1, 5);
-        String fieldName = "";
+        StringBuilder fieldName = new StringBuilder();
         for (int i = 0; i < numLevels; i++) {
             if (i > 0) {
-                fieldName += ".";
+                fieldName.append('.');
             }
-            fieldName += randomString(random);
+            fieldName.append(randomString(random));
         }
-        return fieldName;
+        return fieldName.toString();
     }
 
     /**

test/framework/src/main/java/org/elasticsearch/test/hamcrest/ElasticsearchAssertions.java

@@ -196,21 +196,25 @@ public class ElasticsearchAssertions {
     }
 
     public static String formatShardStatus(BroadcastResponse response) {
-        String msg = " Total shards: " + response.getTotalShards() + " Successful shards: " + response.getSuccessfulShards() + " & "
-                + response.getFailedShards() + " shard failures:";
+        StringBuilder msg = new StringBuilder();
+        msg.append(" Total shards: ").append(response.getTotalShards())
+           .append(" Successful shards: ").append(response.getSuccessfulShards())
+           .append(" & ").append(response.getFailedShards()).append(" shard failures:");
         for (ShardOperationFailedException failure : response.getShardFailures()) {
-            msg += "\n " + failure.toString();
+            msg.append("\n ").append(failure);
         }
-        return msg;
+        return msg.toString();
     }
 
     public static String formatShardStatus(SearchResponse response) {
-        String msg = " Total shards: " + response.getTotalShards() + " Successful shards: " + response.getSuccessfulShards() + " & "
-                + response.getFailedShards() + " shard failures:";
+        StringBuilder msg = new StringBuilder();
+        msg.append(" Total shards: ").append(response.getTotalShards())
+           .append(" Successful shards: ").append(response.getSuccessfulShards())
+           .append(" & ").append(response.getFailedShards()).append(" shard failures:");
         for (ShardSearchFailure failure : response.getShardFailures()) {
-            msg += "\n " + failure.toString();
+            msg.append("\n ").append(failure);
         }
-        return msg;
+        return msg.toString();
     }
 
     public static void assertNoSearchHits(SearchResponse searchResponse) {