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CHAPTER 12

Execution

This chapter specifies activities that occur during execution of a program. It is organized around the life cycle of a Java virtual machine and of the classes, interfaces, and objects that form a program.

A Java virtual machine starts up by loading a specified class and then invoking the method main in this specified class. Section §12.1 outlines the loading, linking, and initialization steps involved in executing main, as an introduction to the concepts in this chapter. Further sections specify the details of loading (§12.2), linking (§12.3), and initialization (§12.4).

The chapter continues with a specification of the procedures for creation of new class instances (§12.5); and finalization of class instances (§12.6). It concludes by describing the unloading of classes (§12.7) and the procedure followed when a program exits (§12.8).

12.1 Virtual Machine Start-Up

The precise semantics of virtual machine start-up are given in chapter 5 of The Java Virtual Machine Specification, Second Edition. Here we present an overview of the process from the viewpoint of the Java programming language.

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

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

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

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 the Java programming language and the Java virtual machine. If a problem is detected during verification, then an error is thrown. Verification is described further in §12.3.1.

Preparation involves allocation of static storage and any data structures that are used internally by the virtual machine, such as method tables. Preparation is described further in §12.3.2.

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 resolution process is described further in §12.3.3.

12.1.3 Initialize Test: Execute Initializers

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 §12.4.

12.1.4 Invoke Test.main

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

public static void main(String[] args)

public static void main(String... args)

12.2 Loading of Classes and Interfaces

The precise semantics of loading are given in chapter 5 of The Java Virtual Machine Specification (whenever we refer to the Java virtual machine specification in this book, we mean the second edition, as amended by JSR 924). Here we present an overview of the process from the viewpoint of the Java programming language.

The binary format of a class or interface is normally the class file format described in The Java Virtual Machine Specification cited above, but other formats are possible, provided they meet the requirements specified in §13.1. The method defineClass of class ClassLoader may be used to construct Class objects from binary representations in the class file format.

Well-behaved class loaders maintain these properties:

• Given the same name, a good class loader should always return the same class object.
• If a class loader L1 delegates loading of a class C to another loader L2, then for any type T that occurs as the direct superclass or a direct superinterface of C, or as the type of a field in C, or as the type of a formal parameter of a method or constructor in C, or as a return type of a method in C, L1 and L2 should return the same class object.

For further discussion of these issues, see The Java Virtual Machine Specification and the paper Dynamic Class Loading in the Java Virtual Machine, by Sheng Liang and Gilad Bracha, in Proceedings of OOPSLA '98, published as ACM SIGPLAN Notices, Volume 33, Number 10, October 1998, pages 36-44. A basic principle of the design of the Java programming language is that the run-time type system cannot be subverted by code written in the language, not even by implementations of such otherwise sensitive system classes as ClassLoader and SecurityManager.

12.2.1 The Loading Process

If an error occurs during class loading, then an instance of one of the following subclasses of class LinkageError will be thrown at any point in the program that (directly or indirectly) uses the type:

• ClassCircularityError: A class or interface could not be loaded because it would be its own superclass or superinterface (§13.4.4).
• ClassFormatError: The binary data that purports to specify a requested compiled class or interface is malformed.
• NoClassDefFoundError: No definition for a requested class or interface could be found by the relevant class loader.

12.3 Linking of Classes and Interfaces

Three different activities are involved in linking: verification, preparation, and resolution of symbolic references.The precise semantics of linking are given in chapter 5 of The Java Virtual Machine Specification, Second Edition. Here we present an overview of the process from the viewpoint of the Java programming language.

This specification allows an implementation flexibility as to when linking activities (and, because of recursion, loading) take place, provided that the semantics of the language are respected, that a class or interface is completely verified and prepared before it is initialized, and that errors detected during linkage are thrown at a point in the program where some action is taken by the program that might require linkage to the class or interface involved in the error.

For example, an implementation may choose to resolve each symbolic reference in a class or interface individually, only when it is used (lazy or late resolution), or to resolve them all at once while the class is being verified (static resolution). This means that the resolution process may continue, in some implementations, after a class or interface has been initialized.

Because linking involves the allocation of new data structures, it may fail with an OutOfMemoryError.

12.3.1 Verification of the Binary Representation

For the specification of the verification process, see the separate volume of this series, The Java Virtual Machine Specification. and the specification of the J2ME Connected Limited Device Configuration, version 1.1.

If an error occurs during verification, then an instance of the following subclass of class LinkageError will be thrown at the point in the program that caused the class to be verified:

• VerifyError: The binary definition for a class or interface failed to pass a set of required checks to verify that it obeys the semantics of the Java virtual machine language and that it cannot violate the integrity of the Java virtual machine. (See §13.4.2, §13.4.4, §13.4.9, and §13.4.17 for some examples.)

12.3.2 Preparation of a Class or Interface Type

Implementations of the Java virtual machine may precompute additional data structures at preparation time in order to make later operations on a class or interface more efficient. One particularly useful data structure is a "method table" or other data structure that allows any method to be invoked on instances of a class without requiring a search of superclasses at invocation time.

12.3.3 Resolution of Symbolic References

Before a symbolic reference can be used it must undergo resolution, wherein a symbolic reference is checked to be correct and, typically, replaced with a direct reference that can be more efficiently processed if the reference is used repeatedly.

If an error occurs during resolution, then an error will be thrown. Most typically, this will be an instance of one of the following subclasses of the class IncompatibleClassChangeError, but it may also be an instance of some other subclass of IncompatibleClassChangeError or even an instance of the class IncompatibleClassChangeError itself. This error may be thrown at any point in the program that uses a symbolic reference to the type, directly or indirectly:

• IllegalAccessError: A symbolic reference has been encountered that specifies a use or assignment of a field, or invocation of a method, or creation of an instance of a class, to which the code containing the reference does not have access because the field or method was declared private, protected, or default access (not public), or because the class was not declared public.

  This can occur, for example, if a field that is originally declared public is changed to be private after another class that refers to the field has been compiled (§13.4.7).

• InstantiationError: A symbolic reference has been encountered that is used in class instance creation expression, but an instance cannot be created because the reference turns out to refer to an interface or to an abstract class.

  This can occur, for example, if a class that is originally not abstract is changed to be abstract after another class that refers to the class in question has been compiled (§13.4.1).

• NoSuchFieldError: A symbolic reference has been encountered that refers to a specific field of a specific class or interface, but the class or interface does not contain a field of that name.

  This can occur, for example, if a field declaration was deleted from a class after another class that refers to the field was compiled (§13.4.8).

• NoSuchMethodError: A symbolic reference has been encountered that refers to a specific method of a specific class or interface, but the class or interface does not contain a method of that signature.

  This can occur, for example, if a method declaration was deleted from a class after another class that refers to the method was compiled (§13.4.12).

12.4 Initialization of Classes and Interfaces

Before a class is initialized, its superclass must be initialized, but interfaces implemented by the class are not initialized. Similarly, the superinterfaces of an interface are not initialized before the interface is initialized.

12.4.1 When Initialization Occurs

Before a class is initialized, its direct superclass must be initialized, but interfaces implemented by the class need not be initialized. Similarly, the superinterfaces of an interface need not be initialized before the interface is initialized.

A class or interface type T will be initialized immediately before the first occurrence of any one of the following:

• T is a class and an instance of T is created.
• T is a class and a static method declared by T is invoked.
• A static field declared by T is assigned.
• A static field declared by T is used and the field is not a constant variable (§4.12.4).
• T is a top-level class, and an assert statement (§14.10) lexically nested within T is executed.

The intent here is that a class or interface type has a set of initializers that put it in a consistent state, and that this state is the first state that is observed by other classes. The static initializers and class variable initializers are executed in textual order, and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope (§8.3.2.3). This restriction is designed to detect, at compile time, most circular or otherwise malformed initializations.

As shown in an example in §8.3.2.3, the fact that initialization code is unrestricted allows examples to be constructed where the value of a class variable can be observed when it still has its initial default value, before its initializing expression is evaluated, but such examples are rare in practice. (Such examples can be also constructed for instance variable initialization; see the example at the end of §12.5). The full power of the language is available in these initializers; programmers must exercise some care. This power places an extra burden on code generators, but this burden would arise in any case because the language is concurrent (§12.4.3).

Before a class is initialized, its superclasses are initialized, if they have not previously been initialized.

Thus, the test program:

class Super {
        static { System.out.print("Super "); }
}
class One {
        static { System.out.print("One "); }
}
class Two extends Super {
        static { System.out.print("Two "); }
}
class Test {
        public static void main(String[] args) {
                One o = null;
                Two t = new Two();
                System.out.println((Object)o == (Object)t);
        }
}

Super Two false

The class One is never initialized, because it not used actively and therefore is never linked to. The class Two is initialized only after its superclass Super has been initialized.

A reference to a class field causes initialization of only the class or interface that actually declares it, even though it might be referred to through the name of a subclass, a subinterface, or a class that implements an interface.

The test program:

class Super { static int taxi = 1729; }
class Sub extends Super {
        static { System.out.print("Sub "); }
}
class Test {
        public static void main(String[] args) {
                System.out.println(Sub.taxi);
        }
}

1729

Initialization of an interface does not, of itself, cause initialization of any of its superinterfaces.

Thus, the test program:

interface I {
        int i = 1, ii = Test.out("ii", 2);
}
interface J extends I {
        int j = Test.out("j", 3), jj = Test.out("jj", 4);
}
interface K extends J {
        int k = Test.out("k", 5);
}
class Test {
        public static void main(String[] args) {
                System.out.println(J.i);
                System.out.println(K.j);
        }
        static int out(String s, int i) {
                System.out.println(s + "=" + i);
                return i;
        }
}

1
j=3
jj=4
3

The reference to J.i is to a field that is a compile-time constant; therefore, it does not cause I to be initialized. The reference to K.j is a reference to a field actually declared in interface J that is not a compile-time constant; this causes initialization of the fields of interface J, but not those of its superinterface I, nor those of interface K. Despite the fact that the name K is used to refer to field j of interface J, interface K is not initialized.

12.4.2 Detailed Initialization Procedure

• This Class object is verified and prepared but not initialized.
• This Class object is being initialized by some particular thread T.
• This Class object is fully initialized and ready for use.
• This Class object is in an erroneous state, perhaps because initialization was attempted and failed.

1. Synchronize (§14.19) on the Class object that represents the class or interface to be initialized. This involves waiting until the current thread can obtain the lock for that object (§17.1).
2. If initialization is in progress for the class or interface by some other thread, then wait on this Class object (which temporarily releases the lock). When the current thread awakens from the wait, repeat this step.
3. If initialization is in progress for the class or interface by the current thread, then this must be a recursive request for initialization. Release the lock on the Class object and complete normally.
4. If the class or interface has already been initialized, then no further action is required. Release the lock on the Class object and complete normally.
5. If the Class object is in an erroneous state, then initialization is not possible. Release the lock on the Class object and throw a NoClassDefFoundError.
6. Otherwise, record the fact that initialization of the Class object is now in progress by the current thread and release the lock on the Class object.
7. Next, if the Class object represents a class rather than an interface, and the superclass of this class has not yet been initialized, then recursively perform this entire procedure for the superclass. If necessary, verify and prepare the superclass first. If the initialization of the superclass completes abruptly because of a thrown exception, then lock this Class object, label it erroneous, notify all waiting threads, release the lock, and complete abruptly, throwing the same exception that resulted from initializing the superclass.
8. Next, determine whether assertions are enabled (§14.10) for this class by querying its defining class loader.
9. Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order, as though they were a single block, except that final class variables and fields of interfaces whose values are compile-time constants are initialized first (§8.3.2.1, §9.3.1, §13.4.9).
10. If the execution of the initializers completes normally, then lock this Class object, label it fully initialized, notify all waiting threads, release the lock, and complete this procedure normally.
11. Otherwise, the initializers must have completed abruptly by throwing some exception E. If the class of E is not Error or one of its subclasses, then create a new instance of the class ExceptionInInitializerError, with E as the argument, and use this object in place of E in the following step. But if a new instance of ExceptionInInitializerError cannot be created because an OutOfMemoryError occurs, then instead use an OutOfMemoryError object in place of E in the following step.
12. Lock the Class object, label it erroneous, notify all waiting threads, release the lock, and complete this procedure abruptly with reason E or its replacement as determined in the previous step.

(Due to a flaw in some early implementations, a exception during class initialization was ignored, rather than causing an ExceptionInInitializerError as described here.)

12.4.3 Initialization: Implications for Code Generation

Compile-time analysis may, in some cases, be able to eliminate many of the checks that a type has been initialized from the generated code, if an initialization order for a group of related types can be determined. Such analysis must, however, fully account for concurrency and for the fact that initialization code is unrestricted.

12.5 Creation of New Class Instances

A new class instance may be implicitly created in the following situations:

• Loading of a class or interface that contains a String literal (§3.10.5) may create a new String object to represent that literal. (This might not occur if the same String has previously been interned (§3.10.5).)
• Execution of an operation that causes boxing conversion (§5.1.7). Boxing conversion may create a new object of a wrapper class associated with one of the primitive types.
• Execution of a string concatenation operator (§15.18.1) that is not part of a constant expression sometimes creates a new String object to represent the result. String concatenation operators may also create temporary wrapper objects for a value of a primitive type.

Whenever a new class instance is created, memory space is allocated for it with room for all the instance variables declared in the class type and all the instance variables declared in each superclass of the class type, including all the instance variables that may be hidden (§8.3). If there is not sufficient space available to allocate memory for the object, then creation of the class instance completes abruptly with an OutOfMemoryError. Otherwise, all the instance variables in the new object, including those declared in superclasses, are initialized to their default values (§4.12.5).

Just before a reference to the newly created object is returned as the result, the indicated constructor is processed to initialize the new object using the following procedure:

1. Assign the arguments for the constructor to newly created parameter variables for this constructor invocation.
2. If this constructor begins with an explicit constructor invocation of another constructor in the same class (using this), then evaluate the arguments and process that constructor invocation recursively using these same five steps. If that constructor invocation completes abruptly, then this procedure completes abruptly for the same reason; otherwise, continue with step 5.
3. This constructor does not begin with an explicit constructor invocation of another constructor in the same class (using this). If this constructor is for a class other than Object, then this constructor will begin with an explicit or implicit invocation of a superclass constructor (using super). Evaluate the arguments and process that superclass constructor invocation recursively using these same five steps. If that constructor invocation completes abruptly, then this procedure completes abruptly for the same reason. Otherwise, continue with step 4.
4. Execute the instance initializers and instance variable initializers for this class, assigning the values of instance variable initializers to the corresponding instance variables, in the left-to-right order in which they appear textually in the source code for the class. If execution of any of these initializers results in an exception, then no further initializers are processed and this procedure completes abruptly with that same exception. Otherwise, continue with step 5. (In some early implementations, the compiler incorrectly omitted the code to initialize a field if the field initializer expression was a constant expression whose value was equal to the default initialization value for its type.)
5. Execute the rest of the body of this constructor. If that execution completes abruptly, then this procedure completes abruptly for the same reason. Otherwise, this procedure completes normally.

In the example:

class Point {
        int x, y;
        Point() { x = 1; y = 1; }
}
class ColoredPoint extends Point {
        int color = 0xFF00FF;
}
class Test {
        public static void main(String[] args) {
                ColoredPoint cp = new ColoredPoint();
                System.out.println(cp.color);
        }
}

ColoredPoint() { super(); }

This constructor then invokes the Point constructor with no arguments. The Point constructor does not begin with an invocation of a constructor, so the compiler provides an implicit invocation of its superclass constructor of no arguments, as though it had been written:

Point() { super(); x = 1; y = 1; }

The class Object has no superclass, so the recursion terminates here. Next, any instance initializers, instance variable initializers of Object are invoked. Next, the body of the constructor of Object that takes no arguments is executed. No such constructor is declared in Object, so the compiler supplies a default one, which in this special case is:

Object() { }

Next, all initializers for the instance variables of class Point are executed. As it happens, the declarations of x and y do not provide any initialization expressions, so no action is required for this step of the example. Then the body of the Point constructor is executed, setting x to 1 and y to 1.

Next, the initializers for the instance variables of class ColoredPoint are executed. This step assigns the value 0xFF00FF to color. Finally, the rest of the body of the ColoredPoint constructor is executed (the part after the invocation of super); there happen to be no statements in the rest of the body, so no further action is required and initialization is complete.

Unlike C++, the Java programming language does not specify altered rules for method dispatch during the creation of a new class instance. If methods are invoked that are overridden in subclasses in the object being initialized, then these overriding methods are used, even before the new object is completely initialized. Thus, compiling and running the example:

class Super {
        Super() { printThree(); }
        void printThree() { System.out.println("three"); }
}
class Test extends Super {
        int three = (int)Math.PI;                                                                                                       // That is, 3
        public static void main(String[] args) {
                Test t = new Test();
                t.printThree();
        }
        void printThree() { System.out.println(three); }
}

produces the output:

0
3

See §8.8 for more details on constructor declarations.

12.6 Finalization of Class Instances

Finalizers provide a chance to free up resources that cannot be freed automatically by an automatic storage manager. In such situations, simply reclaiming the memory used by an object would not guarantee that the resources it held would be reclaimed.

The completion of an object's constructor happens-before (§17.4.5) the execution of its finalize method (in the formal sense of happens-before).

Discussion

It is important to note that many finalizer threads may be active (this is sometimes needed on large shared memory multiprocessors), and that if a large connected data structure becomes garbage, all of the finalize methods for every object in that data structure could be invoked at the same time, each finalizer invocation running in a different thread.

The finalize method declared in class Object takes no action. The fact that class Object declares a finalize method means that the finalize method for any class can always invoke the finalize method for its superclass. This should always be done, unless it is the programmer's intent to nullify the actions of the finalizer in the superclass. (Unlike constructors, finalizers do not automatically invoke the finalizer for the superclass; such an invocation must be coded explicitly.)

For efficiency, an implementation may keep track of classes that do not override the finalize method of class Object, or override it in a trivial way, such as:

protected void finalize() throws Throwable {
        super.finalize();
}

We encourage implementations to treat such objects as having a finalizer that is not overridden, and to finalize them more efficiently, as described in §12.6.1.

The package java.lang.ref describes weak references, which interact with garbage collection and finalization. As with any API that has special interactions with the language, implementors must be cognizant of any requirements imposed by the java.lang.ref API. This specification does not discuss weak references in any way. Readers are referred to the API documentation for details.

12.6.1 Implementing Finalization

A reachable object is any object that can be accessed in any potential continuing computation from any live thread. Optimizing transformations of a program can be designed that reduce the number of objects that are reachable to be less than those which would naively be considered reachable. For example, a compiler or code generator may choose to set a variable or parameter that will no longer be used to null to cause the storage for such an object to be potentially reclaimable sooner.

Discussion

Another example of this occurs if the values in an object's fields are stored in registers. The program may then access the registers instead of the object, and never access the object again. This would imply that the object is garbage.

Note that this sort of optimization is only allowed if references are on the stack, not stored in the heap.

For example, consider the Finalizer Guardian pattern:

class Foo {
    private final Object finalizerGuardian = new Object() {
      protected void finalize() throws Throwable {
        /* finalize outer Foo object */
      }
    }
  } 

If these optimizations are allowed for references that are stored on the heap, then the compiler can detect that the finalizerGuardian field is never read, null it out, collect the object immediately, and call the finalizer early. This runs counter to the intent: the programmer probably wanted to call the Foo finalizer when the Foo instance became unreachable. This sort of transformation is therefore not legal: the inner class object should be reachable for as long as the outer class object is reachable.

Transformations of this sort may result in invocations of the finalize method occurring earlier than might be otherwise expected. In order to allow the user to prevent this, we enforce the notion that synchronization may keep the object alive. If an object's finalizer can result in synchronization on that object, then that object must be alive and considered reachable whenever a lock is held on it.

Note that this does not prevent synchronization elimination: synchronization only keeps an object alive if a finalizer might synchronize on it. Since the finalizer occurs in another thread, in many cases the synchronization could not be removed anyway.

A finalizer-reachable object can be reached from some finalizable object through some chain of references, but not from any live thread. An unreachable object cannot be reached by either means.

An unfinalized object has never had its finalizer automatically invoked; a finalized object has had its finalizer automatically invoked. A finalizable object has never had its finalizer automatically invoked, but the Java virtual machine may eventually automatically invoke its finalizer.

An object o is not finalizable until its constructor has invoked the constructor for Object on o and that invocation has completed successfully (that is, without throwing an exception). Every pre-finalization write to a field of an object must be visible to the finalization of that object. Furthermore, none of the pre-finalization reads of fields of that object may see writes that occur after finalization of that object is initiated.

12.6.1.1 Interaction with the Memory Model

Each execution has a number of reachability decision points, labeled di. Each action either comes-before di or comes-after di. Other than as explicitly mentioned, the comes-before ordering described in this section is unrelated to all other orderings in the memory model.

If r is a read that sees a write w and r comes-before di, then w must come-before di. If x and y are synchronization actions on the same variable or monitor such that so(x, y) (§17.4.4) and y comes-before di, then x must come-before di.

At each reachability decision point, some set of objects are marked as unreachable, and some subset of those objects are marked as finalizable. These reachability decision points are also the points at which references are checked, enqueued and cleared according to the rules provided in the API documentation for the package java.lang.ref.

The only objects that are considered definitely reachable at a point di are those that can be shown to be reachable by the application of these rules:

• An object B is definitely reachable at di from static fields if there exists a write w1 to a static field v of a class C such that the value written by w1 is a reference to B, the class C is loaded by a reachable classloader and there does not exist a write w2 to v such that hb(w2, w1) is not true and both w1 and w2 come-before di.
• An object B is definitely reachable from A at di if there is a write w1 to an element v of A such that the value written by w1 is a reference to B and there does not exist a write w2 to v such that hb(w2, w1) is not true and both w1 and w2 come-before di.
• If an object C is definitely reachable from an object B, and object B is definitely reachable from an object A, then C is definitely reachable from A.

An action a is an active use of X if and only if at least one of the following conditions holds:

• a reads or writes an element of X
• a locks or unlocks X and there is a lock action on X that happens-after the invocation of the finalizer for X.
• a writes a reference to X
• a is an active use of an object Y, and X is definitely reachable from Y

• X must not be definitely reachable at di from static fields,
• All active uses of X in thread t that come-after di must occur in the finalizer invocation for X or as a result of thread t performing a read that comes-after di of a reference to X.
• All reads that come-after di that see a reference to X must see writes to elements of objects that were unreachable at di, or see writes that came after di.

• X must be marked as unreachable at di,
• di must be the only place where X is marked as finalizable,
• actions that happen-after the finalizer invocation must come-after di

12.6.2 Finalizer Invocations are Not Ordered

As an example, if a circularly linked group of unfinalized objects becomes unreachable (or finalizer-reachable), then all the objects may become finalizable together. Eventually, the finalizers for these objects may be invoked, in any order, or even concurrently using multiple threads. If the automatic storage manager later finds that the objects are unreachable, then their storage can be reclaimed.

It is straightforward to implement a class that will cause a set of finalizer-like methods to be invoked in a specified order for a set of objects when all the objects become unreachable. Defining such a class is left as an exercise for the reader.

12.7 Unloading of Classes and Interfaces

Here is the rationale for the rule given in the previous paragraph:

Class unloading is an optimization that helps reduce memory use. Obviously, the semantics of a program should not depend on whether and how a system chooses to implement an optimization such as class unloading. To do otherwise would compromise the portability of programs. Consequently, whether a class or interface has been unloaded or not should be transparent to a program.

However, if a class or interface C was unloaded while its defining loader was potentially reachable, then C might be reloaded. One could never ensure that this would not happen. Even if the class was not referenced by any other currently loaded class, it might be referenced by some class or interface, D, that had not yet been loaded. When D is loaded by C's defining loader, its execution might cause reloading of C.

Reloading may not be transparent if, for example, the class has:

• Static variables (whose state would be lost).
• Static initializers (which may have side effects).

  Native methods (which may retain static state).

Furthermore the hash value of the Class object is dependent on its identity. Therefore it is, in general, impossible to reload a class or interface in a completely transparent manner.

Since we can never guarantee that unloading a class or interface whose loader is potentially reachable will not cause reloading, and reloading is never transparent, but unloading must be transparent, it follows that one must not unload a class or interface while its loader is potentially reachable. A similar line of reasoning can be used to deduce that classes and interfaces loaded by the bootstrap loader can never be unloaded.

One must also argue why it is safe to unload a class C if its defining class loader can be reclaimed. If the defining loader can be reclaimed, then there can never be any live references to it (this includes references that are not live, but might be resurrected by finalizers). This, in turn, can only be true if there are can never be any live references to any of the classes defined by that loader, including C, either from their instances or from code.

Class unloading is an optimization that is only significant for applications that load large numbers of classes and that stop using most of those classes after some time. A prime example of such an application is a web browser, but there are others. A characteristic of such applications is that they manage classes through explicit use of class loaders. As a result, the policy outlined above works well for them.

Strictly speaking, it is not essential that the issue of class unloading be discussed by this specification, as class unloading is merely an optimization. However, the issue is very subtle, and so it is mentioned here by way of clarification.

12.8 Program Exit

• All the threads that are not daemon threads terminate.
• Some thread invokes the exit method of class Runtime or class System and the exit operation is not forbidden by the security manager.

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Java Language Specification Third Edition

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