JVM学习笔记-方法区示例与常量池解析(Method Area Use And Constant Pool Resolution)

 

As an example of how the Java Virtual Machine uses the information it stores in the method area, consider these classes:

为了展示虚拟机如何使用方法区中的信息，我们举个例子，看下面这个类：

begin

// On CD-ROM in file jvm/ex2/Lava.java

class Lava {

 

private int speed = 5; // 5 kilometers per hour

 

void flow() {

}

}

 

 

// On CD-ROM in file jvm/ex2/Volcano.java

class Volcano {

 

public static void main(String[] args) {

Lava lava = new Lava();

lava.flow();

}

}

 

end

 

The following paragraphs describe how an implementation might execute the first instruction in the bytecodes for the main() method of the Volcano application. Different implementations of the Java Virtual Machine can operate in very different ways. The following description illustrates one way--but not the only way--a Java Virtual Machine could execute the first instruction of Volcanoís main() method.

下面的段落描述了某个实现是如何执行Volcano程序中main()方法的字节码中第一条指令的。不同的虚拟机实现可能会用完全不同的方法来操作，下 面描述的只是其中一种可能，但是并不是仅有的一种，下面看一下Java虚拟机是如何执行Volcano程序中main()方法的第一条指令的。

 

To run the Volcano application, you give the name "Volcano" to a Java Virtual Machine in an implementation-dependent manner. Given the name Volcano, the virtual machine finds and reads in file Volcano.class. It extracts the definition of class Volcano from the binary data in the imported class file and places the information into the method area. The virtual machine then invokes the main() method, by interpreting the bytecodes stored in the method area. As the virtual machine executes main(), it maintains a pointer to the constant pool (a data structure in the method area) for the current class (class Volcano).

要运行Volcano程序，首先得以某种“依赖于实现的”方式告诉虚拟机“Volcano”这个名字。之后虚拟机将找到并读入相应的class文件 “Volcano.class”，然后他会从导入的class文件里的二进制数据中提取类型信息并放到方法区中。通过执行保存在方法区中的字节码，虚拟机开始执行main()方法，在执行时，他会一直持有指向当前类（Volcano类）的常量池（方法区中的一个数据结构）的指针。

 

Note that this Java Virtual Machine has already begun to execute the bytecodes for main() in class Volcano even though it hasnít yet loaded class Lava. Like many (probably most) implementations of the Java Virtual Machine, this implementation doesnít wait until all classes used by the application are loaded before it begins executing main(). It loads classes only as it needs them.

注意，虚拟机开始执行Volcano类中main（）方法的字节码的时候，尽管Lava类还没被装载，但是和大多数（也许是所有）虚拟机实现一样，他不会等到把程序中用到的所有类都装载后才开始运行程序。恰好相反，他只需在需要时才装载相应的类 。

 

main()'s first instruction tells the Java Virtual Machine to allocate enough memory for the class listed in constant pool entry one. The virtual machine uses its pointer into Volcanoís constant pool to look up entry one and finds a symbolic reference to class Lava. It checks the method area to see if Lava has already been loaded.

main()的第一条指令告知虚拟机为列在常量池第一项的类分配足够的内存。所以虚拟机使用指向Volcano常量池的指针找到第一项，发现他是一个对Lava类的符号引用，然后他就检查方法区，看Lava类是否已经被装载了。

 

The symbolic reference is just a string giving the classís fully qualified name: "Lava". Here you can see that the method area must be organized so a class can be located--as quickly as possible--given only the classís fully qualified name. Implementation designers can choose whatever algorithm and data structures best fit their needs--a hash table, a search tree, anything. This same mechanism can be used by the static forName() method of class Class, which returns a Class reference given a fully qualified name.

这个符号引用仅仅是一个给出了类Lava的全限定名“Lava”的字符串。为了能让虚拟机尽可能快地从一个名称找到类，设计者应当选择最佳的数据结构和算法。这里可以采用各种方法，如散列表、搜索树等等。同样的算法也可以用于实现Class类的forName（）方法，这个方法根据给定的全限定名返回 Class引用。

 

When the virtual machine discovers that it hasnít yet loaded a class named "Lava," it proceeds to find and read in file Lava.class. It extracts the definition of class Lava from the imported binary data and places the information into the method area.

当虚拟机发现还没有装载过名为“Lava”的类时，他就开始查找并装载文件“Lava.class”，并把从读入的二进制数据中提取的类型信息放在方法区中。

 

The Java Virtual Machine then replaces the symbolic reference in Volcanoís constant pool entry one, which is just the string "Lava", with a pointer to the class data for Lava. If the virtual machine ever has to use Volcanoís constant pool entry one again, it wonít have to go through the relatively slow process of searching through the method area for class Lava given only a symbolic reference, the string "Lava". It can just use the pointer to more quickly access the class data for Lava. This process of replacing symbolic references with direct references (in this case, a native pointer) is called constant pool resolution. The symbolic reference is resolved into a direct reference by searching through the method area until the referenced entity is found, loading new classes if necessary.

紧接着，虚拟机以一个直接指向方法区Lava类数据的指针类替换常量池第一项（就是那个字符串“Lava”）----以后就可以用这个指针来快速访问Lava类了。这个替换过程称为常量池解析 ，即把常量池中的符号引用替换为直接引用。这是通过在方法区中搜索被引用的元素实现的，在这期间可能又需要装载其他类。在这里，我们替换掉符号引用的“直接引用”是一个本地指针。

 

Finally, the virtual machine is ready to actually allocate memory for a new Lava object. Once again, the virtual machine consults the information stored in the method area. It uses the pointer (which was just put into Volcanoís constant pool entry one) to the Lava data (which was just imported into the method area) to find out how much heap space is required by a Lava object.

终于，虚拟机转变为一个新的Lava对象分配内存。此时，它又需要方法区中的信息。还记得刚刚放到Volcano类常量池第一项的指针吗？现在虚拟机用它 来访问Lava类型信息（此前刚放到方法区中的），找到其中记录的这样一个信息：一个Lava对象需要分配多少堆空间。

 

A Java Virtual Machine can always determine the amount of memory required to represent an object by looking into the class data stored in the method area. The actual amount of heap space required by a particular object, however, is implementation-dependent. The internal representation of objects inside a Java Virtual Machine is another decision of implementation designers. Object representation is discussed in more detail later in this chapter.

Java虚拟机总能够通过存储于方法区的类型信息来实现一个对象需要的内存，但是，某一个特定对象事实上需要多少内存，是跟特定实现相关的。对象在虚拟机内部的表示由实现的设计者来决定的。

 

Once the Java Virtual Machine has determined the amount of heap space required by a Lava object, it allocates that space on the heap and initializes the instance variable speed to zero, its default initial value. If class Lavaís superclass, Object, has any instance variables, those are also initialized to default initial values. (The details of initialization of both classes and objects are given in Chapter 7, "The Lifetime of a Class.")

当java虚拟机确定了一个Lava对象的大小后，它就在堆上分配这么大的空间，并把这个对象实例的变量speed初始化为默认初始值0.假如Lava类的超类Object也有实例变量，这也会在此时被初始化为相应的默认值。

 

The first instruction of main() completes by pushing a reference to the new Lava object onto the stack. A later instruction will use the reference to invoke Java code that initializes the speed variable to its proper initial value, five. Another instruction will use the reference to invoke the flow() method on the referenced Lava object.

当把新生成的Lava对象的引用压到栈中，main()方法的第一条指令也完成了。接下来的指令通过这个引用调用Java代码（该代码把speed变量初始化为正确初始值5）。另外一条指令将用这个引用调用Lava对  象引用的flow()方法。