use StringBuilder instead of StringBuffer when can (#1980)

common/src/main/java/com/taobao/arthas/common/ReflectUtils.java

@@ -212,7 +212,7 @@ public class ReflectUtils {
         while ((index = className.indexOf("[]", index) + 1) > 0) {
             dimensions++;
         }
-        StringBuffer brackets = new StringBuffer(className.length() - dimensions);
+        StringBuilder brackets = new StringBuilder(className.length() - dimensions);
         for (int i = 0; i < dimensions; i++) {
             brackets.append('[');
         }

core/src/main/java/com/taobao/arthas/core/util/NetUtils.java

@@ -83,7 +83,7 @@ public class NetUtils {
             int responseCode = con.getResponseCode();
 
             br = new BufferedReader(new InputStreamReader(con.getInputStream()));
-            StringBuffer sb = new StringBuffer();
+            StringBuilder sb = new StringBuilder();
             String line = null;
             while ((line = br.readLine()) != null) {
                 sb.append(line);
@@ -138,7 +138,7 @@ public class NetUtils {
             pw.flush();
 
             br = new BufferedReader(new InputStreamReader(s.getInputStream()));
-            StringBuffer sb = new StringBuffer();
+            StringBuilder sb = new StringBuilder();
             String line = null;
             boolean start = false;
             while ((line = br.readLine()) != null) {

core/src/main/java/com/taobao/arthas/core/util/StringUtils.java

@@ -689,7 +689,7 @@ public abstract class StringUtils {
         }
         int arraySize = array.length;
         int bufSize = (arraySize == 0 ? 0 : (array[0].toString().length() + separator.length()) * arraySize);
-        StringBuffer buf = new StringBuffer(bufSize);
+        StringBuilder buf = new StringBuilder(bufSize);
 
         for (int i = 0; i < arraySize; i++) {
             if (i > 0) {