java/String: add method ToString()

src/java/String.cxx

 /*
- * Copyright 2010-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2010-2019 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -29,6 +29,7 @@
 
 #include "String.hxx"
 #include "util/TruncateString.hxx"
+#include "util/ScopeExit.hxx"
 
 char *
 Java::String::CopyTo(JNIEnv *env, jstring value,
@@ -42,3 +43,20 @@ Java::String::CopyTo(JNIEnv *env, jstring value,
 	env->ReleaseStringUTFChars(value, p);
 	return result;
 }
+
+std::string
+Java::String::ToString(JNIEnv *env, jstring s) noexcept
+{
+	assert(env != nullptr);
+	assert(s != nullptr);
+
+	const char *p = env->GetStringUTFChars(s, nullptr);
+	if (p == nullptr)
+		return std::string();
+
+	AtScopeExit(env, s, p) {
+		env->ReleaseStringUTFChars(s, p);
+	};
+
+	return std::string(p);
+}

src/java/String.hxx

 /*
- * Copyright 2010-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2010-2019 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -34,6 +34,8 @@
 
 #include <jni.h>
 
+#include <string>
+
 #include <stddef.h>
 
 namespace Java {
@@ -69,6 +71,12 @@ namespace Java {
 			     char *buffer, size_t max_size) noexcept {
 			return CopyTo(env, Get(), buffer, max_size);
 		}
+
+		static std::string ToString(JNIEnv *env, jstring s) noexcept;
+
+		std::string ToString() const noexcept {
+			return ToString(GetEnv(), Get());
+		}
 	};
 }