CHAPTER 12: Execution

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12.1 Virtual Machine Start-Up

A Java Virtual Machine starts execution by invoking the method main of some specified class, passing it a single argument, which is an array of strings. In the examples in this specification, this first class is typically called Test .

The manner in which the initial class is specified to the Java Virtual Machine is beyond the scope of this specification, but it is typical, in host environments that use command lines, for the fully-qualified name of the class to be specified as a command-line argument and for following command-line arguments to be used as strings to be provided as the argument to the method main . For example, in a UNIX implementation, the command line:

java Test reboot Bob Dot Enzo

will typically start a Java Virtual Machine by invoking method main of class Test (a class in an unnamed package), passing it an array containing the four strings "reboot" , "Bob" , "Dot" , and "Enzo" .

We now outline the steps the virtual machine may take to execute Test , as an example of the loading, linking, and initialization processes that are described further in later sections.

12.1.1 Load the Class Test

The initial attempt to execute the method main of class Test discovers that the class Test is not loaded-that is, that the virtual machine does not currently contain a binary representation for this class. The virtual machine then uses a class loader (S20.14) to attempt to find such a binary representation. If this process fails, then an error is thrown. This loading process is described further in S12.2.

12.1.2 Link Test: Verify, Prepare, (Optionally) Resolve

After Test is loaded, it must be initialized before main can be invoked. And Test , like all (class or interface) types, must be linked before it is initialized. Linking involves verification, preparation and (optionally) resolution. Linking is described further in S12.3.

Verification checks that the loaded representation of Test is well-formed, with a proper symbol table. Verification also checks that the code that implements Test obeys the semantic requirements of Java and the Java Virtual Machine. If a problem is detected during verification, then an error is thrown. Verification is described further in S12.3.1.

Preparation involves allocation of static storage and any data structures that are used internally by the virtual machine, such as method tables. If a problem is detected during preparation, then an error is thrown. Preparation is described further in S12.3.2.

Resolution is the process of checking symbolic references from Test to other classes and interfaces, by loading the other classes and interfaces that are mentioned and checking that the references are correct.

The resolution step is optional at the time of initial linkage. An implementation may resolve symbolic references from a class or interface that is being linked very early, even to the point of resolving all symbolic references from the classes and interfaces that are further referenced, recursively. (This resolution may result in errors from these further loading and linking steps.) This implementation choice represents one extreme and is similar to the kind of "static" linkage that has been done for many years in simple implementations of the C language. (In these implementations, a compiled program is typically represented as an "a.out " file that contains a fully-linked version of the program, including completely resolved links to library routines used by the program. Copies of these library routines are included in the "a.out " file.)

An implementation may instead choose to resolve a symbolic reference only when it is actively used; consistent use of this strategy for all symbolic references would represent the "laziest" form of resolution. In this case, if Test had several symbolic references to another class, then the references might be resolved one at a time, as they are used, or perhaps not at all, if these references were never used during execution of the program.

The only requirement on when resolution is performed is that any errors detected during resolution must be thrown at a point in the program where some action is taken by the program that might, directly or indirectly, require linkage to the class or interface involved in the error. Using the "static" example implementation choice described above, loading and linkage errors could occur before the program is executed if they involved a class or interface mentioned in the class Test or any of the further, recursively referenced, classes and interfaces. In a system that implemented the "laziest" resolution, these errors would be thrown only when an incorrect symbolic reference is actively used.

The resolution process is described further in S12.3.3.

12.1.3 Initialize Test: Execute Initializers

In our continuing example, the virtual machine is still trying to execute the method main of class Test . This is an attempted active use (S12.4.1) of the class, which is permitted only if the class has been initialized.

Initialization consists of execution of any class variable initializers and static initializers of the class Test , in textual order. But before Test can be initialized, its direct superclass must be initialized, as well as the direct superclass of its direct superclass, and so on, recursively. In the simplest case, Test has Object as its implicit direct superclass; if class Object has not yet been initialized, then it must be initialized before Test is initialized. Class Object has no superclass, so the recursion terminates here.

If class Test has another class Super as its superclass, then Super must be initialized before Test . This requires loading, verifying, and preparing Super if this has not already been done and, depending on the implementation, may also involve resolving the symbolic references from Super and so on, recursively.

Initialization may thus cause loading, linking, and initialization errors, including such errors involving other types.

The initialization process is described further in S12.4.

12.1.4 Invoke Test.main

Finally, after completion of the initialization for class Test (during which other consequential loading, linking, and initializing may have occurred), the method main of Test is invoked.

The method main must be declared public , static , and void . It must accept a single argument that is an array of strings.