J2SE 1.4 premieres Java's assertion capabilities, Part 1

Understand the mechanics of Java's new assertion facility

By Wm. Paul Rogers, JavaWorld.com, 11/09/01

As I will explain in Part 2 of this series, assertions help implement correct programs. Assertions were actually part of Oak, an early version of Java, but were jettisoned in a final push to get Java out of the lab and into the hands of Internet developers. As part of the Java Community Process, Java Specification Request 41 proposed adding a simple assertion facility to Java, prompting its welcome reappearance in J2SE (Java 2 Platform, Standard Edition) version 1.4.

An assertion is a boolean expression that a developer specifically proclaims to be true during program runtime execution. The simple idea of using assertions can have an unexpected influence on a software program's design and implementation. In this article I cover the mechanics of using the new assertion facility introduced in J2SE 1.4. In Part 2 I will cover the methodology of using assertions.

Read the whole series on J2SE 1.4's assertion capabilities:

• Part 1: Understand the mechanics of Java's new assertion facility
• Part 2: Understand the methodology impact of Java's new assertion facility

Declare an assertion

You declare assertions with a new Java language keyword, assert. An assert statement has two permissible forms:

1. assert expression1;
2. assert expression1 : expression2;

In each form, expression1 is the boolean-typed expression being asserted. The expression represents a program condition that the developer specifically proclaims must be true during program execution. In the second form, expression2 provides a means of passing a String message to the assertion facility. The following are a few examples of the first form:

1. assert 0 < value;
2. assert ref != null;
3. assert count == (oldCount + 1);
4. assert ref.m1(parm);

The asserted expression must be of type boolean, which the first three expressions obviously are. In the fourth expression, the method call m1(parm) must return a boolean result. A compile-time error occurs if expression1 does not evaluate to type boolean.

As an example of using assertions, class Foo listed below contains a simple assertion in the method m1(int):

public class Foo
{
  public void m1( int value )
  {
    assert 0 <= value;
    System.out.println( "OK" );
  }
  public static void main( String[] args )
  {
    Foo foo = new Foo();
    System.out.print( "foo.m1(  1 ): " );
    foo.m1( 1 );
    System.out.print( "foo.m1( -1 ): " );
    foo.m1( -1 );
  }
}

The method main() calls m1(int) twice, once with a positive value and once with a negative value. The call with the negative value triggers an assertion error. Since assert is a new Java keyword, to see this example in action, you must compile the class with a J2SE 1.4-compliant compiler. Furthermore, the compiler requires a command-line option, -source 1.4, to signal source compilation using the assertion facility. Requiring a command-line switch to include assertions purportedly protects backward compatibility.

By default (that is, in the absence of the -source 1.4 switch), a J2SE 1.4 compiler does not allow assert statements. However, the compiler complains if the source code uses the assert keyword as an identifier or label. That means a J2SE 1.4 compiler rejects prior Java source files that use assert in this manner, even though the source compiled successfully under J2SE 1.3 or an earlier compiler. Note that this does not affect previously compiled class files.

The following command compiles Foo.java:

javac -source 1.4 Foo.java

The resulting Foo.class file contains assertion code in the method m1(int). But, just as the compiler does not, by default, include the assertion facility, the java command does not, by default, enable assertions. In other words, assertions are disabled in the Java runtime environment by default.

The default behavior of the compiler and runtime system seems backward to me. Assertions are an important enough addition to the Java language that they should be included and enabled by default. The compile-line option should be -source 1.3 for the backward compatibility of not including the assertion facility, and the java command should enable assertions by default. I appreciate the pressure to preserve backward compatibility, but assertions prove too important to be relegated to a special, nondefault case. But I'll get off my soapbox now and continue.

Enable assertions

Command-line options to the java command allow enabling or disabling assertions down to the individual class level. The command-line switch -enableassertions, or -ea for short, enables assertions. The switch (I use the short form) has the following permissible forms:

1. -ea
2. -ea:<class name>
3. -ea:...
4. -ea:<package name>...

The first form enables assertions in all classes except system classes. A separate switch, -enablesystemsassertions, or -esa for short, enables system class assertions. System classes warrant a separate switch because developers rarely have occasion to suspect assertion errors in the Java system libraries.

The second form turns on assertions for the named class only. The last two forms enable assertions at the package level. The third enables assertions for the default, or unnamed, package, and the fourth enables assertions for the specified package name.

Take care in using the second and fourth forms, since class and package names are not verified for existence. As we'll see later in this article, the class ClassLoader maintains a mapping of class and package names to desired assertion status. When a ClassLoader subclass loads a class, the mappings determine the setting of a special assertions-enabled flag in each class. Any mappings for nonexistent classes or packages are simply never accessed. In particular, the runtime system silently interprets a package name without a trailing "..." as a class name.

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