hotspot source code - jdk8u downloads

hotspot 源码下载

hotspot下载链接：http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/archive/tip.tar.gz

或者指定某个版本：http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/archive/1c0a59cee0e4.tar.gz

源码查看

解压下载的文件，使用vscode打开解压后的目录，按vscode提示安装C/C++ for Visual Studio Code插件。

jdk 源码下载

jdk8u下载链接：http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/archive/tip.tar.gz

java native 方法查看

在vscode中打开上面下载的jdk8u源代码，如 Java String#intern()方法的源码即为share/native/java/lang目录下的Java_java_lang_String_intern。

JNIEXPORT jobject JNICALL
Java_java_lang_String_intern(JNIEnv *env, jobject this)
{
    return JVM_InternString(env, this);
}

在openjdk/hotspot/src/share/vm/prims/jvm.cpp文件第4058行，找到JVM_InternString方法定义如下：

// String support ///////////////////////////////////////////////////////////////////////////

JVM_ENTRY(jstring, JVM_InternString(JNIEnv *env, jstring str))
  JVMWrapper("JVM_InternString");
  JvmtiVMObjectAllocEventCollector oam;
  if (str == NULL) return NULL;
  oop string = JNIHandles::resolve_non_null(str);
  oop result = StringTable::intern(string, CHECK_NULL);
  return (jstring) JNIHandles::make_local(env, result);
JVM_END

上面代码中的StringTable::intern()方法是关键代码，这个方法定义在openjdk/hotspot/src/share/vm/classfile/symbolTable.cpp第730行，代码如下：

oop StringTable::lookup(jchar* name, int len) {
  unsigned int hash = hash_string(name, len);
  int index = the_table()->hash_to_index(hash);
  oop string = the_table()->lookup(index, name, len, hash);

  ensure_string_alive(string);

  return string;
}


oop StringTable::intern(Handle string_or_null, jchar* name,
                        int len, TRAPS) {
  unsigned int hashValue = hash_string(name, len);
  int index = the_table()->hash_to_index(hashValue);
  oop found_string = the_table()->lookup(index, name, len, hashValue);

  // Found
  if (found_string != NULL) {
    ensure_string_alive(found_string);
    return found_string;
  }

  debug_only(StableMemoryChecker smc(name, len * sizeof(name[0])));
  assert(!Universe::heap()->is_in_reserved(name),
         "proposed name of symbol must be stable");

  Handle string;
  // try to reuse the string if possible
  if (!string_or_null.is_null()) {
    string = string_or_null;
  } else {
    string = java_lang_String::create_from_unicode(name, len, CHECK_NULL);
  }

#if INCLUDE_ALL_GCS
  if (G1StringDedup::is_enabled()) {
    // Deduplicate the string before it is interned. Note that we should never
    // deduplicate a string after it has been interned. Doing so will counteract
    // compiler optimizations done on e.g. interned string literals.
    G1StringDedup::deduplicate(string());
  }
#endif

  // Grab the StringTable_lock before getting the_table() because it could
  // change at safepoint.
  oop added_or_found;
  {
    MutexLocker ml(StringTable_lock, THREAD);
    // Otherwise, add to symbol to table
    added_or_found = the_table()->basic_add(index, string, name, len,
                                  hashValue, CHECK_NULL);
  }

  ensure_string_alive(added_or_found);

  return added_or_found;
}

oop StringTable::intern(Symbol* symbol, TRAPS) {
  if (symbol == NULL) return NULL;
  ResourceMark rm(THREAD);
  int length;
  jchar* chars = symbol->as_unicode(length);
  Handle string;
  oop result = intern(string, chars, length, CHECK_NULL);
  return result;
}


oop StringTable::intern(oop string, TRAPS)
{
  if (string == NULL) return NULL;
  ResourceMark rm(THREAD);
  int length;
  Handle h_string (THREAD, string);
  jchar* chars = java_lang_String::as_unicode_string(string, length, CHECK_NULL);
  oop result = intern(h_string, chars, length, CHECK_NULL);
  return result;
}


oop StringTable::intern(const char* utf8_string, TRAPS) {
  if (utf8_string == NULL) return NULL;
  ResourceMark rm(THREAD);
  int length = UTF8::unicode_length(utf8_string);
  jchar* chars = NEW_RESOURCE_ARRAY(jchar, length);
  UTF8::convert_to_unicode(utf8_string, chars, length);
  Handle string;
  oop result = intern(string, chars, length, CHECK_NULL);
  return result;
}

void StringTable::unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* f, int* processed, int* removed) {
  BucketUnlinkContext context;
  buckets_unlink_or_oops_do(is_alive, f, 0, the_table()->table_size(), &context);
  _the_table->bulk_free_entries(&context);
  *processed = context._num_processed;
  *removed = context._num_removed;
}

References

1. jdk8u / hotspot
2. jdk8u/jdk
3. vm/prims/jvm.cpp
4. vm/classfile/symbolTable.cpp