realm java 源码疑问

JNIEXPORT void JNICALL Java_io_realm_internal_Group_nativeWriteToFile(
    JNIEnv* env, jobject, jlong nativeGroupPtr, jstring jFileName, jbyteArray keyArray)
{
    TR_ENTER_PTR(nativeGroupPtr)
    StringData file_name;
    KeyBuffer key(env, keyArray);
    try {
        JStringAccessor file_name_tmp(env, jFileName); // throws
        file_name = StringData(file_name_tmp);
#ifdef REALM_ENABLE_ENCRYPTION
        G(nativeGroupPtr)->write(file_name, key.data());
#else
        G(nativeGroupPtr)->write(file_name);
#endif
    }
    CATCH_FILE(file_name)
    CATCH_STD()
}

#define G(x)    reinterpret_cast<realm::Group*>(x)

    /**
     * Serializes the group to the specific file on the disk using encryption.
     *
     * @param file a File object representing the file.
     * @param key A 64 bytes long byte array containing the key to the encrypted Realm file. Can be null if encryption
     *            is not required.
     * @throws IOException.
     */
    public void writeToFile(File file, byte[] key) throws IOException {
        verifyGroupIsValid();
        if (file.isFile() && file.exists()) {
            throw new IllegalArgumentException("The destination file must not exist");
        }
        if (key != null && key.length != 64) {
            throw new IllegalArgumentException("Realm AES keys must be 64 bytes long");
        }

        nativeWriteToFile(nativePtr, file.getAbsolutePath(), key);
    }

    public Group() {
        this.immutable = false;
        this.context = new Context();
        this.nativePtr = createNative();
        checkNativePtrNotZero();
    }

JNIEXPORT jlong JNICALL Java_io_realm_internal_Group_createNative__(
    JNIEnv*,  jobject)
{
    TR_ENTER()
    Group *ptr = new Group();
    TR("Group::createNative(): %p.", VOID_PTR(ptr))
    return reinterpret_cast<jlong>(ptr);
}

#include <realm/util/safe_int_ops.hpp>