[Android] Avoid unnecessary JNI calls for empty string conversions

base/android/jni_string.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/android/jni_string.h"
 
 #include "base/android/jni_android.h"
 #include "base/logging.h"
 #include "base/strings/utf_string_conversions.h"
 
@@ skipping 11 matching lines @@
 
 namespace base {
 namespace android {
 
 void ConvertJavaStringToUTF8(JNIEnv* env, jstring str, std::string* result) {
   if (!str) {
     LOG(WARNING) << "ConvertJavaStringToUTF8 called with null string.";
     result->clear();
     return;
   }
+  const jsize length = env->GetStringLength(str);
+  if (!length) {
+    result->clear();
+    return;

[nyquist, 2015/08/04 18:05:29]

Do we need CheckException(env) here, since we're using the environment? (same
below)

[jdduke (slow), 2015/08/04 18:10:44]

Ohh good catch, done.

+  }
   // JNI's GetStringUTFChars() returns strings in Java "modified" UTF8, so
   // instead get the String in UTF16 and convert using chromium's conversion
   // function that yields plain (non Java-modified) UTF8.
   const jchar* chars = env->GetStringChars(str, NULL);
   DCHECK(chars);
-  UTF16ToUTF8(chars, env->GetStringLength(str), result);
+  UTF16ToUTF8(chars, length, result);
   env->ReleaseStringChars(str, chars);
   CheckException(env);
 }
 
 std::string ConvertJavaStringToUTF8(JNIEnv* env, jstring str) {
   std::string result;
   ConvertJavaStringToUTF8(env, str, &result);
   return result;
 }
 
@@ skipping 14 matching lines @@
   return ScopedJavaLocalRef<jstring>(env, ConvertUTF16ToJavaStringImpl(
       env, UTF8ToUTF16(str)));
 }
 
 void ConvertJavaStringToUTF16(JNIEnv* env, jstring str, string16* result) {
   if (!str) {
     LOG(WARNING) << "ConvertJavaStringToUTF16 called with null string.";
     result->clear();
     return;
   }
+  const jsize length = env->GetStringLength(str);
+  if (!length) {
+    result->clear();
+    return;
+  }
   const jchar* chars = env->GetStringChars(str, NULL);
   DCHECK(chars);
   // GetStringChars isn't required to NULL-terminate the strings
   // it returns, so the length must be explicitly checked.
-  result->assign(chars, env->GetStringLength(str));
+  result->assign(chars, length);
   env->ReleaseStringChars(str, chars);
   CheckException(env);
 }
 
 string16 ConvertJavaStringToUTF16(JNIEnv* env, jstring str) {
   string16 result;
   ConvertJavaStringToUTF16(env, str, &result);
   return result;
 }
 
 string16 ConvertJavaStringToUTF16(const JavaRef<jstring>& str) {
   return ConvertJavaStringToUTF16(AttachCurrentThread(), str.obj());
 }
 
 ScopedJavaLocalRef<jstring> ConvertUTF16ToJavaString(
     JNIEnv* env,
     const base::StringPiece16& str) {
   return ScopedJavaLocalRef<jstring>(env,
                                      ConvertUTF16ToJavaStringImpl(env, str));
 }
 
 }  // namespace android
 }  // namespace base