From ce7611562f6e89773f6f9c6cea7e6de963b1fe89 Mon Sep 17 00:00:00 2001
From: Peter Johnson <johnson.peter@gmail.com>
Date: Wed, 14 Dec 2016 23:16:09 -0800
Subject: [PATCH] Make JClass more useful and use it in ntcore JNI.

---
 java/lib/NetworkTablesJNI.cpp      | 68 +++++++++++-------------------
 wpiutil/include/support/jni_util.h |  8 ++--
 2 files changed, 29 insertions(+), 47 deletions(-)

diff --git a/java/lib/NetworkTablesJNI.cpp b/java/lib/NetworkTablesJNI.cpp
index 03c77eb..1dd91f9 100644
--- a/java/lib/NetworkTablesJNI.cpp
+++ b/java/lib/NetworkTablesJNI.cpp
@@ -24,13 +24,13 @@ using namespace wpi::java;
 
 // Used for callback.
 static JavaVM *jvm = nullptr;
-static jclass booleanCls = nullptr;
-static jclass doubleCls = nullptr;
-static jclass connectionInfoCls = nullptr;
-static jclass entryInfoCls = nullptr;
-static jclass keyNotDefinedEx = nullptr;
-static jclass persistentEx = nullptr;
-static jclass illegalArgEx = nullptr;
+static JClass booleanCls;
+static JClass doubleCls;
+static JClass connectionInfoCls;
+static JClass entryInfoCls;
+static JClass keyNotDefinedEx;
+static JClass persistentEx;
+static JClass illegalArgEx;
 // Thread-attached environment for listener callbacks.
 static JNIEnv *listenerEnv = nullptr;
 
@@ -64,49 +64,29 @@ JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {
     return JNI_ERR;
 
   // Cache references to classes
-  jclass local;
-
-  local = env->FindClass("java/lang/Boolean");
-  if (!local) return JNI_ERR;
-  booleanCls = static_cast<jclass>(env->NewGlobalRef(local));
+  booleanCls = JClass(env, "java/lang/Boolean");
   if (!booleanCls) return JNI_ERR;
-  env->DeleteLocalRef(local);
 
-  local = env->FindClass("java/lang/Double");
-  if (!local) return JNI_ERR;
-  doubleCls = static_cast<jclass>(env->NewGlobalRef(local));
+  doubleCls = JClass(env, "java/lang/Double");
   if (!doubleCls) return JNI_ERR;
-  env->DeleteLocalRef(local);
 
-  local = env->FindClass("edu/wpi/first/wpilibj/networktables/ConnectionInfo");
-  if (!local) return JNI_ERR;
-  connectionInfoCls = static_cast<jclass>(env->NewGlobalRef(local));
+  connectionInfoCls =
+      JClass(env, "edu/wpi/first/wpilibj/networktables/ConnectionInfo");
   if (!connectionInfoCls) return JNI_ERR;
-  env->DeleteLocalRef(local);
 
-  local = env->FindClass("edu/wpi/first/wpilibj/networktables/EntryInfo");
-  if (!local) return JNI_ERR;
-  entryInfoCls = static_cast<jclass>(env->NewGlobalRef(local));
+  entryInfoCls = JClass(env, "edu/wpi/first/wpilibj/networktables/EntryInfo");
   if (!entryInfoCls) return JNI_ERR;
-  env->DeleteLocalRef(local);
 
-  local =
-      env->FindClass("edu/wpi/first/wpilibj/networktables/NetworkTableKeyNotDefined");
-  keyNotDefinedEx = static_cast<jclass>(env->NewGlobalRef(local));
+  keyNotDefinedEx = JClass(
+      env, "edu/wpi/first/wpilibj/networktables/NetworkTableKeyNotDefined");
   if (!keyNotDefinedEx) return JNI_ERR;
-  env->DeleteLocalRef(local);
 
-  local =
-      env->FindClass("edu/wpi/first/wpilibj/networktables/PersistentException");
-  persistentEx = static_cast<jclass>(env->NewGlobalRef(local));
+  persistentEx =
+      JClass(env, "edu/wpi/first/wpilibj/networktables/PersistentException");
   if (!persistentEx) return JNI_ERR;
-  env->DeleteLocalRef(local);
 
-  local = env->FindClass("java/lang/IllegalArgumentException");
-  if (!local) return JNI_ERR;
-  illegalArgEx = static_cast<jclass>(env->NewGlobalRef(local));
+  illegalArgEx = JClass(env, "java/lang/IllegalArgumentException");
   if (!illegalArgEx) return JNI_ERR;
-  env->DeleteLocalRef(local);
 
   // Initial configuration of listener start/exit
   nt::SetListenerOnStart(ListenerOnStart);
@@ -120,13 +100,13 @@ JNIEXPORT void JNICALL JNI_OnUnload(JavaVM *vm, void *reserved) {
   if (vm->GetEnv(reinterpret_cast<void **>(&env), JNI_VERSION_1_6) != JNI_OK)
     return;
   // Delete global references
-  if (booleanCls) env->DeleteGlobalRef(booleanCls);
-  if (doubleCls) env->DeleteGlobalRef(doubleCls);
-  if (connectionInfoCls) env->DeleteGlobalRef(connectionInfoCls);
-  if (entryInfoCls) env->DeleteGlobalRef(entryInfoCls);
-  if (keyNotDefinedEx) env->DeleteGlobalRef(keyNotDefinedEx);
-  if (persistentEx) env->DeleteGlobalRef(persistentEx);
-  if (illegalArgEx) env->DeleteGlobalRef(illegalArgEx);
+  booleanCls.free(env);
+  doubleCls.free(env);
+  connectionInfoCls.free(env);
+  entryInfoCls.free(env);
+  keyNotDefinedEx.free(env);
+  persistentEx.free(env);
+  illegalArgEx.free(env);
   jvm = nullptr;
 }
 
diff --git a/wpiutil/include/support/jni_util.h b/wpiutil/include/support/jni_util.h
index 2564c13..7659abd 100644
--- a/wpiutil/include/support/jni_util.h
+++ b/wpiutil/include/support/jni_util.h
@@ -32,7 +32,9 @@ namespace java {
 // to potential shutdown issues with doing so.
 class JClass {
  public:
-  JClass(JNIEnv* env, const char* name) : m_cls(nullptr) {
+  JClass() = default;
+
+  JClass(JNIEnv* env, const char* name) {
     jclass local = env->FindClass(name);
     if (!local) return;
     m_cls = static_cast<jclass>(env->NewGlobalRef(local));
@@ -40,7 +42,7 @@ class JClass {
   }
 
   void free(JNIEnv *env) {
-    env->DeleteGlobalRef(m_cls);
+    if (m_cls) env->DeleteGlobalRef(m_cls);
     m_cls = nullptr;
   }
 
@@ -49,7 +51,7 @@ class JClass {
   operator jclass() const { return m_cls; }
 
  private:
-  jclass m_cls;
+  jclass m_cls = nullptr;
 };
 
 // Container class for cleaning up Java local references.