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Java主类的装载

在JavaMain()函数（定义在openjdk/jdk/src/share/bin/java.c文件中）中调用LoadMainClass()函数加载Java主类。LoadMainClass()函数的实现如下：

源代码位置：openjdk/jdk/src/share/bin/java.c

/*
 * Loads a class and verifies that the main class is present and it is ok to
 * call it for more details refer to the java implementation.
 */
static jclass LoadMainClass(JNIEnv *env, int mode, char *name){

    jmethodID   mid;
    jstring     str;
    jobject     result;
    jlong       start, end;

    // 加载sun.launcher.LauncherHelper类
    jclass cls = GetLauncherHelperClass(env);

    // 获取sun.launcher.LauncherHelper类中定义的checkAndLoadMain()方法的指针
    NULL_CHECK0(mid = (*env)->GetStaticMethodID(env,cls,"checkAndLoadMain","(ZILjava/lang/String;)Ljava/lang/Class;"));

    // 调用sun.launcher.LauncherHelper类中的checkAndLoadMain()方法
    str = NewPlatformString(env, name);
    result = (*env)->CallStaticObjectMethod(env, cls, mid, USE_STDERR, mode, str);

    return (jclass)result;
}

下面介绍如上函数调用的一些函数。　

1、GetLauncherHelperClass()函数

调用的GetLauncherHelperClass()函数的实现如下：

jclass GetLauncherHelperClass(JNIEnv *env){
    if (helperClass == NULL) {
        NULL_CHECK0(helperClass = FindBootStrapClass(env,"sun/launcher/LauncherHelper"));
    }
    return helperClass;
}


/*
 * The implementation for finding classes from the bootstrap
 * class loader, refer to java.h
 */
static FindClassFromBootLoader_t *findBootClass = NULL;

// 参数classname的值为"sun/launcher/LauncherHelper"。
jclass FindBootStrapClass(JNIEnv *env, const char* classname){
   if (findBootClass == NULL) {
       // 返回指向JVM_FindClassFromBootLoader()函数的函数指针
       findBootClass = (FindClassFromBootLoader_t *)dlsym(RTLD_DEFAULT,"JVM_FindClassFromBootLoader"); 
   }
   return findBootClass(env, classname);
}

通过函数指针findBootClass来调用JVM_FindClassFromBootLoader()函数。JVM_FindClassFromBootLoader()函数的实现如下：

JVM_ENTRY(jclass, JVM_FindClassFromBootLoader(JNIEnv* env,const char* name))

  TempNewSymbol h_name = SymbolTable::new_symbol(name, CHECK_NULL);
  Klass* k = SystemDictionary::resolve_or_null(h_name, CHECK_NULL);
  if (k == NULL) {
    return NULL;
  }

  return (jclass) JNIHandles::make_local(env, k->java_mirror());
JVM_END

调用的SystemDictionary::resolve_or_null()函数在前面已经详细介绍过，这里不再介绍。

2、GetStaticMethodID()函数

在通过JNI的方式调用Java方法时，首先要获取到方法的methodID。调用GetStaticMethodID()函数查找Java启动方法（Java主类中的main()方法）的methodID。调用GetStaticMethodID()函数其实调用的是jni_GetStaticMethodID()函数，实现如下：

// 传递的参数name为"checkAndLoadMain"，而sig为"(ZILjava/lang/String;)Ljava/lang/Class;"。
JNI_ENTRY(jmethodID, jni_GetStaticMethodID(JNIEnv *env, jclass clazz,const char *name, const char *sig))
  jmethodID ret = get_method_id(env, clazz, name, sig, true, thread);
  return ret;
JNI_END

get_method_id()函数的实现如下：

static jmethodID get_method_id(JNIEnv *env, jclass clazz, const char *name_str,
                               const char *sig, bool is_static, TRAPS) {
  // %%%% This code should probably just call into a method in the LinkResolver
  //
  // The class should have been loaded (we have an instance of the class
  // passed in) so the method and signature should already be in the symbol
  // table.  If they're not there, the method doesn't exist.
  const char *name_to_probe = (name_str == NULL)
                        ? vmSymbols::object_initializer_name()->as_C_string()
                        : name_str;
  TempNewSymbol name = SymbolTable::probe(name_to_probe, (int)strlen(name_to_probe));
  TempNewSymbol signature = SymbolTable::probe(sig, (int)strlen(sig));

  KlassHandle klass(THREAD,java_lang_Class::as_Klass(JNIHandles::resolve_non_null(clazz)));

  // Make sure class is linked and initialized before handing id's out to
  // Method*s.
  klass()->initialize(CHECK_NULL);

  Method* m;
  if (name == vmSymbols::object_initializer_name() || // name为<init>
      name == vmSymbols::class_initializer_name()) {  // name为<clinit>
    // Never search superclasses for constructors
    if (klass->oop_is_instance()) { // 在查找构造函数时，只查找当前类中的构造函数，不查找超类构造函数
      m = InstanceKlass::cast(klass())->find_method(name, signature);
    } else {
      m = NULL;
    }
  } else {
    m = klass->lookup_method(name, signature); // 在特定类中查找方法
    if (m == NULL &&  klass->oop_is_instance()) {
      m = InstanceKlass::cast(klass())->lookup_method_in_ordered_interfaces(name, signature);
    }
  }

  return m->jmethod_id(); // 获取方法对应的methodID，methodID指定后不会变，所以可以重复使用methodID
}

查找构造函数时调用InstanceKlass类中的find_method()方法，这个方法不会查找超类；查找普通方法时，调用Klass中的lookup_method()或InstanceKlass类中的lookup_method_in_ordered_interfaces()方法，这两个方法会从父类中查找，例如lookup_method()方法的实现如下：

Method* lookup_method(Symbol* name, Symbol* signature) const {
    return uncached_lookup_method(name, signature);
}

// uncached_lookup_method searches both the local class methods array and all
// superclasses methods arrays, skipping any overpass methods in superclasses.
Method* InstanceKlass::uncached_lookup_method(Symbol* name, Symbol* signature) const {
  Klass* klass = const_cast<InstanceKlass*>(this);
  bool dont_ignore_overpasses = true;  // For the class being searched, find its overpasses.
  while (klass != NULL) {
    Method* method = InstanceKlass::cast(klass)->find_method(name, signature);
    if ((method != NULL) && (dont_ignore_overpasses || !method->is_overpass())) {
      return method;
    }
    klass = InstanceKlass::cast(klass)->super();
    dont_ignore_overpasses = false;  // Ignore overpass methods in all superclasses.
  }
  return NULL;
}

如果调用find_method()无法从当前类中查找到对应的方法，那么通过while循环一直从继承链往上查找，如果找到就直接返回，否则返回NULL。

find_method()方法的实现如下：

// find_method looks up the name/signature in the local methods array
Method* InstanceKlass::find_method(Symbol* name, Symbol* signature) const {
  return InstanceKlass::find_method(methods(), name, signature);
}

// find_method looks up the name/signature in the local methods array
Method* InstanceKlass::find_method(Array<Method*>* methods, Symbol* name, Symbol* signature) {
  int hit = find_method_index(methods, name, signature);
  return hit >= 0 ? methods->at(hit): NULL;
}

// Used directly for default_methods to find the index into the
// default_vtable_indices, and indirectly by find_method
// find_method_index looks in the local methods array to return the index
// of the matching name/signature
int InstanceKlass::find_method_index(Array<Method*>* methods, Symbol* name, Symbol* signature) {
  int hit = binary_search(methods, name); // 从methods中通过二分算法来查找名称为name的方法
  if (hit != -1) {
    Method* m = methods->at(hit);
    // Do linear search to find matching signature.  First, quick check for common case
    if (m->signature() == signature)
    	return hit;

    // search downwards through overloaded methods
    int i;
    for (i = hit - 1; i >= 0; --i) {
        Method* m = methods->at(i);
        if (m->name() != name)
        	break;
        if (m->signature() == signature)
        	return i;
    }

    // search upwards
    for (i = hit + 1; i < methods->length(); ++i) {
        Method* m = methods->at(i);
        if (m->name() != name)
        	break;
        if (m->signature() == signature)
        	return i;
    }
    // not found
  }

  return -1;
}

当前的方法存储在instanceKlass类的_methods属性中，并且是按一定的顺序存储，这样就可以使用二分查找算法加快查找速度了，如果找到方法，则返回对应在数组中的下标位置，否则返回-1。

3、CallStaticObjectMethod()函数

在LoadMainClass()函数中调用(*env)->CallStaticObjectMethod()函数会执行sun.launcher.LauncherHelper类的checkAndLoadMain()方法。CallStaticObjectMethod()方法定义在jni.cpp文件中，实现时会通过jni_invoke_static()函数执行checkAndLoadMain()方法。jni_invoke_static()函数的实现如下：

static void jni_invoke_static(JNIEnv *env, JavaValue* result, jobject receiver, JNICallType call_type,
		jmethodID method_id, JNI_ArgumentPusher *args, TRAPS) {
  methodHandle method(THREAD, Method::resolve_jmethod_id(method_id));

  // Create object to hold arguments for the JavaCall, and associate it with
  // the jni parser
  ResourceMark rm(THREAD);
  int number_of_parameters = method->size_of_parameters();
  // 这里进一步将要传给Java的参数转换为JavaCallArguments对象传下去
  JavaCallArguments java_args(number_of_parameters);
  args->set_java_argument_object(&java_args);

  assert(method->is_static(), "method should be static");

  // Fill out JavaCallArguments object
  args->iterate( Fingerprinter(method).fingerprint() );
  // Initialize result type
  result->set_type(args->get_ret_type());

  // Invoke the method. Result is returned as oop.
  // 供C/C++程序调用Java方法
  JavaCalls::call(result, method, &java_args, CHECK);

  // Convert result
  if (result->get_type() == T_OBJECT || result->get_type() == T_ARRAY) {
     result->set_jobject(JNIHandles::make_local(env, (oop) result->get_jobject()));
  }
}

函数最终通过JavaCalls::call()方法调用Java方法，在介绍方法执行引擎时将会详细介绍。方法看起来逻辑很多，其实都是因为JNI调用时，需要对参数进行转换，在JNI环境下只能使用句柄来访问虚拟机对象，而在虚拟机操作时，由于实现逻辑的需要，必须要操作虚拟机对象，所以不可避免在每次方法的开始和结束都需要对参数进行转换。如调用Method::resolve_jmethod_id()、调用JNIHandles::make_local()等方法。　　

最后看一下调用JavaCalls::call()方法执行的Java方法checkAndLoadMain()方法的实现，如下：

源代码位置如下：/home/mazhi/workspace/openjdk/jdk/src/share/classes/sun/launcher/LauncherHelper.java

/**
     * This method does the following:
     * 1. gets the classname from a Jar's manifest, if necessary
     * 2. loads the class using the System ClassLoader
     * 3. ensures the availability and accessibility of the main method,
     *    using signatureDiagnostic method.
     *    a. does the class exist
     *    b. is there a main
     *    c. is the main public
     *    d. is the main static
     *    e. does the main take a String array for args
     * 4. if no main method and if the class extends FX Application, then call
     *    on FXHelper to determine the main class to launch
     * 5. and off we go......
     *
     * @param printToStderr if set, all output will be routed to stderr
     * @param mode LaunchMode as determined by the arguments passed on the
     * command line
     * @param what either the jar file to launch or the main class when using
     * LM_CLASS mode
     * @return the application's main class
     */
    public static Class<?> checkAndLoadMain(boolean printToStderr,
                                            int mode,
                                            String what) {
        initOutput(printToStderr);
        // get the class name
        String cn = null;
        switch (mode) { 
            case LM_CLASS:  
                cn = what;
                break;
            case LM_JAR:
                cn = getMainClassFromJar(what);
                break;
            default:
                // should never happen
                throw new InternalError("" + mode + ": Unknown launch mode");
        }
        cn = cn.replace('/', '.');
        Class<?> mainClass = null;
        try {
            mainClass = scloader.loadClass(cn); // 加载主类
        } catch (NoClassDefFoundError | ClassNotFoundException cnfe) {
            ...
        }
        // set to mainClass
        appClass = mainClass;

        return mainClass;
    }

从这里可以看出加载main方法类的加载器是系统类加载器，而系统类加载器其实就是AppClassLoader。所以，main方法默认加载器是AppClassLoder，并且传给当前线程上下文的加载器也是AppClassLoader。

AppClassLoader/ExtClassLoader都是Launcher的内部类。先初始化ExtClassLoader，并将ExtClassLoader作为父加载器传给AppClassLoder

scloader是全局变量，定义如下：

private static final ClassLoader scloader = ClassLoader.getSystemClassLoader();

调用scloader的loadClass()方法会调用到java.lang.ClassLoader的loadClass()方法，之前已经介绍过这个方法，首先通过findLoadedClass()方法判断当前加载器是否已经加载了指定的类，如果没有加载并且parent不为NULL，调用parent.loadClass()方法来完成，而AppClassLoader的父加载器是ExtClassLoader，这是加载JDK中的扩展类，并不会加载Java主类，所以只能调用this.findClass()方法来完成主类的加载。对于AppClassLoader来说，调用的是URLClassLoader中实现的findClass()方法，最终会调用本地方法defineClass1()来完成，这个方法在介绍类的双亲委派机制时详细介绍过，这里不再介绍。