Introduce JSON_MAGIC_STRING as a common static value

Build the RestApiServlet.JSON_MAGIC encoded as a string and reuse it as
a common static value throughout the code, avoiding repetition and
runtime string builds.

Change-Id: I3c0df8fb20fcf52a06d4e74fba69f1833a5e1462

src/main/java/com/googlesource/gerrit/plugins/healthcheck/HealthCheckExceptionHook.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/HealthCheckExceptionHook.java b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/HealthCheckExceptionHook.java
index 81a3dfc..bb95d22 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/HealthCheckExceptionHook.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/HealthCheckExceptionHook.java
@@ -16,6 +16,7 @@
 
 import static com.google.gerrit.httpd.restapi.RestApiServlet.JSON_MAGIC;
 import static com.googlesource.gerrit.plugins.healthcheck.filter.HealthCheckStatusFilter.GSON;
+import static com.googlesource.gerrit.plugins.healthcheck.filter.HealthCheckStatusFilter.JSON_MAGIC_STRING;
 import static javax.servlet.http.HttpServletResponse.SC_INTERNAL_SERVER_ERROR;
 
 import com.google.gerrit.extensions.registration.DynamicSet;
@@ -32,7 +33,7 @@
       return Optional.of(
           Status.create(
               SC_INTERNAL_SERVER_ERROR,
-              new String(JSON_MAGIC, StandardCharsets.UTF_8) + GSON.toJson(exc.getResult())));
+              JSON_MAGIC_STRING + GSON.toJson(exc.getResult())));
     }
     return Optional.empty();
   }

src/main/java/com/googlesource/gerrit/plugins/healthcheck/filter/HealthCheckStatusFilter.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/filter/HealthCheckStatusFilter.java b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/filter/HealthCheckStatusFilter.java
index 994da24..cb2a58a 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/healthcheck/filter/HealthCheckStatusFilter.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/healthcheck/filter/HealthCheckStatusFilter.java
@@ -29,6 +29,7 @@
 import com.googlesource.gerrit.plugins.healthcheck.api.HealthCheckStatusEndpoint;
 import java.io.IOException;
 import java.io.PrintWriter;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
@@ -43,6 +44,7 @@
 
 public class HealthCheckStatusFilter extends AllRequestFilter {
   public static final Gson GSON = OutputFormat.JSON.newGsonBuilder().create();
+  public static final String JSON_MAGIC_STRING = new String(RestApiServlet.JSON_MAGIC, StandardCharsets.UTF_8);
 
   private final HealthCheckStatusEndpoint statusEndpoint;
   private final String pluginName;
@@ -133,7 +135,7 @@
       String healthStatusJson = GSON.toJson(healthStatus.value());
       if (healthStatus.statusCode() == HttpServletResponse.SC_OK) {
         PrintWriter writer = httpResponse.getWriter();
-        writer.print(new String(RestApiServlet.JSON_MAGIC));
+        writer.print(JSON_MAGIC_STRING);
         writer.print(healthStatusJson);
       } else {
         httpResponse.sendError(healthStatus.statusCode(), healthStatusJson);