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| Contents | Prev | Next | Index | Java Language Specification Third Edition |
CHAPTER 13
Development tools for the Java programming language should support automatic recompilation as necessary whenever source code is available. Particular implementations may also store the source and binary of types in a versioning database and implement a ClassLoader that uses integrity mechanisms of the database to prevent linkage errors by providing binary-compatible versions of types to clients.
Developers of packages and classes that are to be widely distributed face a different set of problems. In the Internet, which is our favorite example of a widely distributed system, it is often impractical or impossible to automatically recompile the pre-existing binaries that directly or indirectly depend on a type that is to be changed. Instead, this specification defines a set of changes that developers are permitted to make to a package or to a class or interface type while preserving (not breaking) compatibility with existing binaries.
The paper quoted above appears in Proceedings of OOPSLA '95, published as ACM SIGPLAN Notices, Volume 30, Number 10, October 1995, pages 426-438. Within the framework of that paper, Java programming language binaries are binary compatible under all relevant transformations that the authors identify (with some caveats with respect to the addition of instance variables). Using their scheme, here is a list of some important binary compatible changes that the Java programming language supports:
private fields, methods, or constructors of a class.
We encourage development systems to provide facilities that alert developers to the impact of changes on pre-existing binaries that cannot be recompiled.
This chapter first specifies some properties that any binary format for the Java programming language must have (§13.1). It next defines binary compatibility, explaining what it is and what it is not (§13.2). It finally enumerates a large set of possible changes to packages (§13.3), classes (§13.4) and interfaces (§13.5), specifying which of these changes are guaranteed to preserve binary compatibility and which are not.
class file format specified by the The Java Virtual Machine Specification, or into a representation that can be mapped into that format by a class loader written in the Java programming language. Furthermore, the resulting class file must have certain properties. A number of these properties are specifically chosen to support source code transformations that preserve binary compatibility.
super.f then the superclass of C is the qualifying type of the reference.
super.f then the superclass of X is the qualifying type of the reference.
The reference to f must be compiled into a symbolic reference to the erasure (§4.6) of the qualifying type of the reference, plus the simple name of the field, f. The reference must also include a symbolic reference to the erasure of the declared type of the field so that the verifier can check that the type is as expected.
If D is Object then the qualifying type of the expression is Object. Otherwise:
super.m then the superclass of C is the qualifying type of the method invocation.
super.m then the superclass of X is the qualifying type of the method invocation.
A reference to a method must be resolved at compile time to a symbolic reference to the erasure (§4.6) of the qualifying type of the invocation, plus the erasure of the signature of the method (§8.4.2). A reference to a method must also include either a symbolic reference to the erasure of the return type of the denoted method or an indication that the denoted method is declared void and does not return a value. The signature of a method must include all of the following:
new D(...) or X.new D(...), then the qualifying type of the invocation is D.
new D(..){...} or X.new D(...){...}, then the qualifying type of the expression is the compile-time type of the expression.
super(...) or Primary.super(...) then the qualifying type of the expression is the direct superclass of C.
this(...), then the qualifying type of the expression is C.
A reference to a constructor must be resolved at compile time to a symbolic reference to the erasure (§4.6) of the qualifying type of the invocation, plus the signature of the constructor (§8.8.2). The signature of a constructor must include both:
In addition the constructor of a non-private inner member class must be compiled such that it has as its first parameter, an additional implicit parameter representing the immediately enclosing instance (§8.1.3).
Object, then a symbolic reference to the erasure of the direct superclass of this class
class file format support these changes. Any other valid binary format, such as a compressed or encrypted representation that is mapped back into class files by a class loader under the above requirements will necessarily support these changes as well. Binaries are compiled to rely on the accessible members and constructors of other classes and interfaces. To preserve binary compatibility, a class or interface should treat its accessible members and constructors, their existence and behavior, as a contract with its users.
The Java programming language is designed to prevent additions to contracts and accidental name collisions from breaking binary compatibility; specifically:
Changes in top-level class and interface types that are not public and that are not a superclass or superinterface, respectively, of a public type, affect only types within the package in which they are declared. Such types may be deleted or otherwise changed, even if incompatibilities are otherwise described here, provided that the affected binaries of that package are updated together.
abstract is changed to be declared abstract, then pre-existing binaries that attempt to create new instances of that class will throw either an InstantiationError at link time, or (if a reflective method is used) an InstantiationException at run time; such a change is therefore not recommended for widely distributed classes.
Changing a class that was declared abstract to no longer be declared abstract does not break compatibility with pre-existing binaries.
final is changed to be declared final, then a VerifyError is thrown if a binary of a pre-existing subclass of this class is loaded, because final classes can have no subclasses; such a change is not recommended for widely distributed classes.
Changing a class that was declared final to no longer be declared final does not break compatibility with pre-existing binaries.
public to be declared public does not break compatibility with pre-existing binaries.
If a class that was declared public is changed to not be declared public, then an IllegalAccessError is thrown if a pre-existing binary is linked that needs but no longer has access to the class type; such a change is not recommended for widely distributed classes.
ClassCircularityError is thrown at load time if a class would be a superclass of itself. Changes to the class hierarchy that could result in such a circularity when newly compiled binaries are loaded with pre-existing binaries are not recommended for widely distributed classes.Changing the direct superclass or the set of direct superinterfaces of a class type will not break compatibility with pre-existing binaries, provided that the total set of superclasses or superinterfaces, respectively, of the class type loses no members.
If a change to the direct superclass or the set of direct superinterfaces results in any class or interface no longer being a superclass or superinterface, respectively, then link-time errors may result if pre-existing binaries are loaded with the binary of the modified class. Such changes are not recommended for widely distributed classes.
For example, suppose that the following test program:
class Hyper { char h = 'h'; }
class Super extends Hyper { char s = 's'; }
class Test extends Super {
public static void printH(Hyper h) {
System.out.println(h.h);
}
public static void main(String[] args) {
printH(new Super());
}
}
Suppose that a new version of classh
Super is then compiled:
class Super { char s = 's'; }
Super is not a subclass of Hyper. If we then run the existing binaries of Hyper and Test with the new version of Super, then a VerifyError is thrown at link time. The verifier objects because the result of new Super() cannot be passed as an argument in place of a formal parameter of type Hyper, because Super is not a subclass of Hyper.It is instructive to consider what might happen without the verification step: the program might run and print:
This demonstrates that without the verifier the type system could be defeated by linking inconsistent binary files, even though each was produced by a correct Java compiler.s
The lesson is that an implementation that lacks a verifier or fails to use it will not maintain type safety and is, therefore, not a valid implementation.
Discussion
Note that if such type variables are used in the type of a field or method, that may have the normal implications of changing the aforementioned type.
Changing the first bound of a type parameter will change the erasure (§4.6) of any member that uses that type variable in its own type, and this may effect binary compatibility. Changing any other bound has no effect on binary compatibility.
static) member that has the same name, accessibility, (for fields) or same name, accessibility, signature, and return type (for methods) as an instance (respectively static) member of a superclass or subclass. No error occurs even if the set of classes being linked would encounter a compile-time error.
Deleting a class member or constructor that is not declared private may cause a linkage error if the member or constructor is used by a pre-existing binary.
If the program:
class Hyper {
void hello() { System.out.println("hello from Hyper"); }
}
class Super extends Hyper {
void hello() { System.out.println("hello from Super"); }
}
class Test {
public static void main(String[] args) {
new Super().hello();
}
}
Suppose that a new version of classhello from Super
Super is produced:
class Super extends Hyper { }
Super and executing this new binary with the original binaries for Test and Hyper produces the output:
as expected.hello from Hyper
The super keyword can be used to access a method declared in a superclass, bypassing any methods declared in the current class. The expression:
is resolved, at compile time, to a method M in the superclass S. If the method M is an instance method, then the method MR invoked at run time is the method with the same signature as M that is a member of the direct superclass of the class containing the expression involvingsuper.Identifier
super. Thus, if the program:
class Hyper {
void hello() { System.out.println("hello from Hyper"); }
}
class Super extends Hyper { }
class Test extends Super {
public static void main(String[] args) {
new Test().hello();
}
void hello() {
super.hello();
}
}
Suppose that a new version of classhello from Hyper
Super is produced:
class Super extends Hyper {
void hello() { System.out.println("hello from Super"); }
}
Super and Hyper are recompiled but not Test, then running the new binaries with the existing binary of Test produces the output:
as you might expect. (A flaw in some early implementations caused them to print:hello from Super
incorrectly.)hello from Hyper
private access; from protected access to default or private access; or from public access to protected, default, or private access. Changing a member or constructor to permit less access is therefore not recommended for widely distributed classes.Perhaps surprisingly, the binary format is defined so that changing a member or constructor to be more accessible does not cause a linkage error when a subclass (already) defines a method to have less access.
So, for example, if the package points defines the class Point:
package points;
public class Point {
public int x, y;
protected void print() {
System.out.println("(" + x + "," + y + ")");
}
}
class Test extends points.Point {
protected void print() {
System.out.println("Test");
}
public static void main(String[] args) {
Test t = new Test();
t.print();
}
}
Test executes to produce the output:
If the methodTest
print in class Point is changed to be public, and then only the Point class is recompiled, and then executed with the previously existing binary for Test then no linkage error occurs, even though it is improper, at compile time, for a public method to be overridden by a protected method (as shown by the fact that the class Test could not be recompiled using this new Point class unless print were changed to be public.)Allowing superclasses to change protected methods to be public without breaking binaries of preexisting subclasses helps make binaries less fragile. The alternative, where such a change would cause a linkage error, would create additional binary incompatibilities.
Assume a reference to a field f with qualifying type T. Assume further that f is in fact an instance (respectively static) field declared in a superclass of T, S, and that the type of f is X. If a new field of type X with the same name as f is added to a subclass of S that is a superclass of T or T itself, then a linkage error may occur. Such a linkage error will occur only if, in addition to the above, either one of the following conditions hold:
static (respectively instance) field.
Thus compiling and executing the code:
class Hyper { String h = "hyper"; }
class Super extends Hyper { String s = "super"; }
class Test {
public static void main(String[] args) {
System.out.println(new Super().h);
}
}
Changinghyper
Super to be defined as:
class Super extends Hyper {
String s = "super";
int h = 0;
}
Hyper and Super, and executing the resulting new binaries with the old binary of Test produces the output:
The fieldhyper
h of Hyper is output by the original binary of main. While this may seem surprising at first, it serves to reduce the number of incompatibilities that occur at run time. (In an ideal world, all source files that needed recompilation would be recompiled whenever any one of them changed, eliminating such surprises. But such a mass recompilation is often impractical or impossible, especially in the Internet. And, as was previously noted, such recompilation would sometimes require further changes to the source code.)As an example, if the program:
class Hyper { String h = "Hyper"; }
class Super extends Hyper { }
class Test extends Super {
public static void main(String[] args) {
String s = new Test().h;
System.out.println(s);
}
}
Suppose that a new version of classHyper
Super is then compiled:
class Super extends Hyper { char h = 'h'; }
Hyper and Test, then the output is still:
even though compiling the source for these binaries:Hyper
class Hyper { String h = "Hyper"; }
class Super extends Hyper { char h = 'h'; }
class Test extends Super {
public static void main(String[] args) {
String s = new Test().h;
System.out.println(s);
}
}
h in the source code for main would now be construed as referring to the char field declared in Super, and a char value can't be assigned to a String.
Deleting a field from a class will break compatibility with any pre-existing binaries that reference this field, and a NoSuchFieldError will be thrown when such a reference from a pre-existing binary is linked. Only private fields may be safely deleted from a widely distributed class.
For purposes of binary compatibility, adding or removing a field f whose type involves type variables (§4.4) or parameterized types (§4.5) is equivalent to the addition (respectively, removal) of a field of the same name whose type is the erasure (§4.6) of the type of f.
final is changed to be final, then it can break compatibility with pre-existing binaries that attempt to assign new values to the field. For example, if the program:
class Super { static char s; }
class Test extends Super {
public static void main(String[] args) {
s = 'a';
System.out.println(s);
}
}
Suppose that a new version of classa
Super is produced:
class Super { final static char s = 'b'; }
Super is recompiled but not Test, then running the new binary with the existing binary of Test results in a IllegalAccessError.
Deleting the keyword final or changing the value to which a field is initialized does not break compatibility with existing binaries.
If a field is a constant variable (§4.12.4), then deleting the keyword final or changing its value will not break compatibility with pre-existing binaries by causing them not to run, but they will not see any new value for the usage of the field unless they are recompiled. This is true even if the usage itself is not a compile-time constant expression (§15.28)
If the example:
class Flags { final static boolean debug = true; }
class Test {
public static void main(String[] args) {
if (Flags.debug)
System.out.println("debug is true");
}
}
Suppose that a new version of classdebug is true
Flags is produced:
class Flags { final static boolean debug = false; }
Flags is recompiled but not Test, then running the new binary with the existing binary of Test produces the output:
because the value ofdebug is true
debug was a compile-time constant, and could have been used in compiling Test without making a reference to the class Flags.This result is a side-effect of the decision to support conditional compilation, as discussed at the end of §14.21.
This behavior would not change if Flags were changed to be an interface, as in the modified example:
interface Flags { boolean debug = true; }
class Test {
public static void main(String[] args) {
if (Flags.debug)
System.out.println("debug is true");
}
}
switch statements require constants on each case, and no two such constant values may be the same. The compiler checks for duplicate constant values in a switch statement at compile time; the class file format does not do symbolic linkage of case values.)The best way to avoid problems with "inconstant constants" in widely-distributed code is to declare as compile time constants only values which truly are unlikely ever to change. Other than for true mathematical constants, we recommend that source code make very sparing use of class variables that are declared static and final. If the read-only nature of final is required, a better choice is to declare a private static variable and a suitable accessor method to get its value. Thus we recommend:
private static int N;
public static int getN() { return N; }
There is no problem with:public static final int N = ...;
ifpublic static int N = ...;
N need not be read-only. We also recommend, as a general rule, that only truly constant values be declared in interfaces. We note, but do not recommend, that if a field of primitive type of an interface may change, its value may be expressed idiomatically as in:
interface Flags {
boolean debug = new Boolean(true).booleanValue();
}
One other thing to note is that static final fields that have constant values (whether of primitive or String type) must never appear to have the default initial value for their type (§4.12.5). This means that all such fields appear to be initialized first during class initialization (§8.3.2.1, §9.3.1, §12.4.2).
private was not declared static and is changed to be declared static, or vice versa, then a linkage time error, specifically an IncompatibleClassChangeError, will result if the field is used by a preexisting binary which expected a field of the other kind. Such changes are not recommended in code that has been widely distributed.transient modifier of a field does not break compatibility with pre-existing binaries.
Assume a reference to a method m with qualifying type T. Assume further that m is in fact an instance (respectively static) method declared in a superclass of T, S. If a new method of type X with the same signature and return type as m is added to a subclass of S that is a superclass of T or T itself, then a linkage error may occur. Such a linkage error will occur only if, in addition to the above, either one of the following conditions hold:
static (respectively instance) method.
NoSuchMethodError may be thrown when such a reference from a pre-existing binary is linked. Such an error will occur only if no method with a matching signature and return type is declared in a superclass. If the source code for a class contains no declared constructors, the Java compiler automatically supplies a constructor with no parameters. Adding one or more constructor declarations to the source code of such a class will prevent this default constructor from being supplied automatically, effectively deleting a constructor, unless one of the new constructors also has no parameters, thus replacing the default constructor. The automatically supplied constructor with no parameters is given the same access modifier as the class of its declaration, so any replacement should have as much or more access if compatibility with pre-existing binaries is to be preserved.
Discussion
Note that if such type variables are used in the type of the method or constructor, that may have the normal implications of changing the aforementioned type.
Changing the first bound of a type parameter may change the erasure (§4.6) of any member that uses that type variable in its own type, and this may effect binary compatibility. Specifically:
For purposes of binary compatibility, adding or removing a method or constructor m whose signature involves type variables (§4.4) or parameterized types (§4.5) is equivalent to the addition (respectively, removal) of an otherwise equivalent method whose signature is the erasure (§4.6) of the signature of m.
void, or replacing void with a result type has the combined effect of deleting the old method and adding a new method with the new result type or newly void result (see §13.4.12).For purposes of binary compatibility, adding or removing a method or constructor m whose return type involves type variables (§4.4) or parameterized types (§4.5) is equivalent to the addition (respectively, removal) of the an otherwise equivalent method whose return type is the erasure (§4.6) of the return type of m.
abstract to no longer be declared abstract does not break compatibility with pre-existing binaries.
Changing a method that is not declared abstract to be declared abstract will break compatibility with pre-existing binaries that previously invoked the method, causing an AbstractMethodError.
If the example program:
class Super { void out() { System.out.println("Out"); } }
class Test extends Super {
public static void main(String[] args) {
Test t = new Test();
System.out.println("Way ");
t.out();
}
}
Suppose that a new version of classWay Out
Super is produced:
abstract class Super {
abstract void out();
}
Super is recompiled but not Test, then running the new binary with the existing binary of Test results in a AbstractMethodError, because class Test has no implementation of the method out, and is therefore is (or should be) abstract. final to be final may break compatibility with existing binaries that depend on the ability to override the method. If the test program:
class Super { void out() { System.out.println("out"); } }
class Test extends Super {
public static void main(String[] args) {
Test t = new Test();
t.out();
}
void out() { super.out(); }
}
Suppose that a new version of classout
Super is produced:
class Super { final void out() { System.out.println("!"); } }
Super is recompiled but not Test, then running the new binary with the existing binary of Test results in a VerifyError because the class Test improperly tries to override the instance method out.
Changing a class (static) method that is not final to be final does not break compatibility with existing binaries, because the method could not have been overridden.
Removing the final modifier from a method does not break compatibility with pre-existing binaries.
native modifier of a method does not break compatibility with pre-existing binaries.
The impact of changes to types on preexisting native methods that are not recompiled is beyond the scope of this specification and should be provided with the description of an implementation. Implementations are encouraged, but not required, to implement native methods in a way that limits such impact.
private was declared static (that is, a class method) and is changed to not be declared static (that is, to an instance method), or vice versa, then compatibility with pre-existing binaries may be broken, resulting in a linkage time error, namely an IncompatibleClassChangeError, if these methods are used by the pre-existing binaries. Such changes are not recommended in code that has been widely distributed.synchronized modifier of a method does not break compatibility with existing binaries.throws clause of methods or constructors do not break compatibility with existing binaries; these clauses are checked only at compile time.We note that a compiler cannot expand a method inline at compile time.
The keyword final on a method does not mean that the method can be safely inlined; it means only that the method cannot be overridden. It is still possible that a new version of that method will be provided at link time. Furthermore, the structure of the original program must be preserved for purposes of reflection.
In general we suggest that implementations use late-bound (run-time) code generation and optimization.
While adding a new overloaded method or constructor may cause a compile-time error the next time a class or interface is compiled because there is no method or constructor that is most specific (§15.12.2.5), no such error occurs when a program is executed, because no overload resolution is done at execution time.
If the example program:
class Super {
static void out(float f) { System.out.println("float"); }
}
class Test {
public static void main(String[] args) {
Super.out(2);
}
}
Suppose that a new version of classfloat
Super is produced:
class Super {
static void out(float f) { System.out.println("float"); }
static void out(int i) { System.out.println("int"); }
}
Super is recompiled but not Test, then running the new binary with the existing binary of Test still produces the output:
However, iffloat
Test is then recompiled, using this new Super, the output is then:
as might have been naively expected in the previous case.int
If a precompiled binary attempts to access an enum constant that no longer exists, the client will fail at runtime with a NoSuchFieldError. Therefore such a change is not recommended for widely distributed enums.
In all other respects, the binary compatibility rules for enums are identical to those for classes.
public to be declared public does not break compatibility with pre-existing binaries.
If an interface that is declared public is changed to not be declared public, then an IllegalAccessError is thrown if a pre-existing binary is linked that needs but no longer has access to the interface type, so such a change is not recommended for widely distributed interfaces.
VerifyError.IncompatibleClassChangeError will result. If the original reference was an assignment, an IllegalAccessError will result.Deleting a member from an interface may cause linkage errors in pre-existing binaries.
If the example program:
interface I { void hello(); }
class Test implements I {
public static void main(String[] args) {
I anI = new Test();
anI.hello();
}
public void hello() { System.out.println("hello"); }
}
Suppose that a new version of interfacehello
I is compiled:
interface I { }
I is recompiled but not Test, then running the new binary with the existing binary for Test will result in a NoSuchMethodError. (In some early implementations this program still executed; the fact that the method hello no longer exists in interface I was not correctly detected.)static final fields in classes, as described in §13.4.8 and §13.4.9.abstract methods in classes, as described in §13.4.14, §13.4.15, §13.4.21, and §13.4.23.Adding or removing annotations has no effect on the correct linkage of the binary representations of programs in the Java programming language.
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