java方法句柄-----4.你所不知道的MethodHandle【翻译】

Method Handles in Java

1.介绍

在本文中，我们将探讨一个重要的API，它是在Java 7中引入的，并在Java 7版本之后更加完善：全限定名是：Java.lang.invoke.MethodHandles。
我们将学习什么是method handles,如何创建它们和如何使用他们。

2.什么是MethodHandle

MethodHandle来自Oracle java7的定义：

"A method handle is a typed, directly executable reference to an underlying method, constructor, field, or similar low-level operation, with optional transformations of arguments or return values. These transformations are quite general, and include such patterns as conversion, insertion, deletion, and substitution. "

这个定义暗示着：MethodHandle:

• 方法的引用。和c++/c中不大相同的是，是对引用的方法的限制：即引用的方法和MethodHandle的type(参数和返回值)必须一样。
• Executable。An Methodhandle can be executed by invoking its methods: invokeExact..
• Method Type transformations.
  简单地说，方法句柄是low-level mechanism(为了查找、调整和调用方法)。
  a low-level mechanism for finding, adapting and invoking methods.

创建和使用一个MethodHandle,需要进行一下四个步骤：

• Creating the lookup // 创建lookup
• Creating the method type // 创建method type
• Finding the method handle // 找到method handle
• Invoking the method handle // 调用

3. Method Handles vs Reflection

Method Handles的引入是为了与已经存在的java.lang.reflect API相配合。他们分别是为了解决不同的问题而出现的。从性能角度上说，MethodHandle api要比反射快很多因为访问检查在创建的时候就已经完成了，而不是像反射一样等到运行时候才检查。但同时，Method Handles比反射更难用，因为没有列举类中成员，获取属性访问标志之类的机制。
另外，MethodHandles可以操作方法，更改方法参数的类型和他们的顺序。而反射则没有这些功能。
从以上角度看，反射更通用，但是安全性更差，因为可以在不授权的情况下使用反射对象。而method Handles遵从了分享者的能力。所以method handle是一种更低级的发现，适配和调用方法的方式，唯一的优点就是更快。所以反射更适合主流Java开发者，而method handle更适用于对编译和运行性能有要求的人。

4.创建method handle

要使用method handle，首先需要得到Lookup。这是创造方法，构造函数，属性的method handles的工厂类。

// public方法的Lookup
MethodHandles.Lookup publicLookup = MethodHandles.publicLookup();
// 所有方法的Lookup
MethodHandles.Lookup lookup = MethodHandles.lookup();

5.创建MethodType

要创建MethodHandle，lookup需要一个定义了它的类型的MethodType对象。这里的类型包括了传入参数的类型，和最后返回的类型，要一一对应。第一个是返回类型，如果没有返回值就是Void.class, 后面是可变的传入参数的类型。

a MethodType represents the arguments and return type accepted and returned by a method handle or passed and expected by a method handle caller.

such as

// 接收数组，返回一个List对象，其中Object[].class作为input type 即入参类型
MethodType mt = MethodType.methodType(List.class, Object[].class);

6.查找MethodHandle

Lookup之所以叫Lookup自然是因为他们有查找MethodHandle的能力。先看看他的方法。

Once we've defined our method type, in order to create a MethodHandle, we have to find it through the lookup or publicLookup object, providing also the origin class and the method name.

In particular, the lookup factory provides a set of methods that allow us to find the method handle in an appropriate way considering the scope of our method.

6.1 Method Handle for Methods

Using the findVirtual() method allow us to create a MethodHandle for an object method. Let's create one, based on the concat() method of the String class:

// 上面说过第一个入参为返回类型，后面是可变的传入参数的类型。
MethodType mt = MethodType.methodType(String.class, String.class);

// 使用lookup的findVirtual函数可以获得一个对象方法的MethodHandle 。查找 the concat() method of the String class
MethodHandle concatMH = publicLookup.findVirtual(String.class, "concat", mt);

6.2.Method Handle for Static Methods

When we want to gain access to a static method, we can instead use the findStatic() method:

MethodType mt = MethodType.methodType(List.class, Object[].class);

MethodHandle asListMH = publicLookup.findStatic(Arrays.class, "asList", mt);

In this case, we created a method handle that converts an array of Objects to a List of them.

6.3. Method Handle for Constructors

Gaining access to a constructor can be done using the findConstructor() method.

Let's create a method handles that behaves as the constructor of the Integer class, accepting a String attribute:

MethodType mt = MethodType.methodType(void.class, String.class);

MethodHandle newIntegerMH = publicLookup.findConstructor(Integer.class, mt);

6.4.Method Handle for Fields

Using a method handle it's possible to gain access also to fields.

Let's start defining the Book class:

public class Book {
     
    String id;
    String title;
 
    // constructor
 
}

Havinga as precondition direct access visibility between the method handle and the declared property , we can create a method handle that behaves as a getter:

MethodHandle getTitleMH = lookup.findGetter(Book.class, "title", String.class);

For further information on handling variables/fields, give a look at the Java 9 Variable Handles Demystified, where we discuss the java.lang.invoke.VarHandle API, added in Java 9.

6.5. Method Handle for Private Methods

Creating a method handle for a private method can be done, with the help of the java.lang.reflect API.

Let's start adding a private method to the Book class:

private String formatBook() {
    return id + " > " + title;
}

Now we can create a method handle that behaves exactly as the formatBook() method:

Method formatBookMethod = Book.class.getDeclaredMethod("formatBook");
formatBookMethod.setAccessible(true);
MethodHandle formatBookMH = lookup.unreflect(formatBookMethod);

7. Invoking a Method Handle -- 方法句柄的调用

在获取到了一个方法句柄之后，最直接的使用方法就是调用它所引用的底层方法。在这点上，方法句柄的使用类似于反射API中的Method类。但是方法句柄在调用时所提供的灵活性是Method类中的invoke方法所不能比的。

Once we've created our method handles, use them is the next step. In particular, the MethodHandle class provides 3 different way to execute a method handle: invoke(), invokeWithArugments() and invokeExact().

7.1. Invoking a Method Handle

When using the invoke() method, we enforce the number of the arguments (arity) to be fixed
(我们强制了固定参数的数量)but we allow the performing of casting and boxing/unboxing of the arguments and return types（但是我们允许对参数和返回类型进行强制转换和装箱/拆箱。）.

Let's see how it's possible to use the invoke() with a boxed argument:

MethodType mt = MethodType.methodType(String.class, char.class, char.class);
MethodHandle replaceMH = publicLookup.findVirtual(String.class, "replace", mt);

String output = (String) replaceMH.invoke("jovo", Character.valueOf('o'), 'a');
assertEquals("java", output);

In this case, the replaceMH requires char arguments, the invoke() performs an unboxing on the Character argument before its execution.

7.2. Invoking With Arguments

Invoking a method handle using the invokeWithArguments method, is the least restrictive of the three options.

In fact, it allows a variable arity invocation, in addition to the casting and boxing/unboxing of the arguments and of the return types.

Coming to practice, this allows us to create a List of Integer starting from an array of int values:

MethodType mt = MethodType.methodType(List.class, Object[].class);

MethodHandle asList = publicLookup.findStatic(Arrays.class, "asList", mt);
 
List<Integer> list = (List<Integer>) asList.invokeWithArguments(1,2);
 
assertThat(Arrays.asList(1,2), is(list));

7.3. Invoking Exact

In case we want to be more restrictive in the way we execute a method handle (number of arguments and their type), we have to use the invokeExact() method.

In fact, it doesn't provide any casting to the class provided and requires a fixed number of arguments.

Let's see how we can sum two int values using a method handle:

MethodType mt = MethodType.methodType(int.class, int.class, int.class);
MethodHandle sumMH = lookup.findStatic(Integer.class, "sum", mt);
 
int sum = (int) sumMH.invokeExact(1, 11);
 
assertEquals(12, sum);

If in this case, we decide to pass to the invokeExact method a number that isn't an int, the invocation will lead to WrongMethodTypeException.

方法调用细则：

有三种方法可以调用方法invoke(), invokeWithArugments()和invokeExact()，当我们使用invoke时，我们必须固定arguments的数目。

具体的区别是：
与invokeExact方法不同，invoke方法允许更加松散的调用方式。它会尝试在调用的时候进行返回值和参数类型的转换工作
这是通过MethodHandle类的asType方法来完成的，asType方法的作用是把当前方法句柄适配到新的MethodType上面，并产生一个新的方法句柄。当方法句柄在调用时的类型与其声明的类型完全一致的时候，调用invoke方法等于调用invokeExact方法；否则，invoke方法会先调用asType方法来尝试适配到调用时的类型。如果适配成功，则可以继续调用。否则会抛出相关的异常。这种灵活的适配机制，使invoke方法成为在绝大多数情况下都应该使用的方法句柄调用方式。

参考：
https://www.baeldung.com/java-method-handles
https://www.cnblogs.com/night-wind/p/4405564.html
https://blog.csdn.net/lyj1597374034/article/details/105250459

参考资料：《java程序员修炼之道》、《深入理解java7核心技术与最佳实践》