Reproducing “too many constants” Problem in Java

In my previous blog post, I blogged on the “code too large” problem and reproduced that error message. In this post, I look at the very similar “too many constants” error message (not the same thing as the question too many constants?) and demonstrate reproducing it by having too many methods in a generated Java class.

With a few small adaptations, I can adjust the Groovy script that I used to generate a Java class to reproduce the “code too large” error to instead generate a Java class to reproduce the “too many constants” error. Here is the revised script.

generateJavaClassWithManyMethods.groovy


#!/usr/bin/env groovy

import javax.tools.ToolProvider

println "You're running the script ${System.getProperty('script.name')}"
if (args.length < 2)
{
println "Usage: javaClassGenerationWithManyMethods packageName className baseDir #methods"
System.exit(-1)
}

// No use of "def" makes the variable available to entire script including the
// defined methods ("global" variables)

packageName = args[0]
packagePieces = packageName.tokenize(".") // Get directory names
def fileName = args[1].endsWith(".java") ? args[1] : args[1] + ".java"
def baseDirectory = args.length > 2 ? args[2] : System.getProperty("user.dir")
numberOfMethods = args.length > 3 ? Integer.valueOf(args[3]) : 10

NEW_LINE = System.getProperty("line.separator")

// The setting up of the indentations shows off Groovy's easy feature for
// multiplying Strings and Groovy's tie of an overloaded * operator for Strings
// to the 'multiply' method. In other words, the "multiply" and "*" used here
// are really the same thing.
SINGLE_INDENT = ' '
DOUBLE_INDENT = SINGLE_INDENT.multiply(2)
TRIPLE_INDENT = SINGLE_INDENT * 3

def outputDirectoryName = createDirectories(baseDirectory)
def generatedJavaFile = generateJavaClass(outputDirectoryName, fileName)
compileJavaClass(generatedJavaFile)


/**
* Generate the Java class and write its source code to the output directory
* provided and with the file name provided. The generated class's name is
* derived from the provided file name.
*
* @param outDirName Name of directory to which to write Java source.
* @param fileName Name of file to be written to output directory (should include
* the .java extension).
* @return Fully qualified file name of source file.
*/
def String generateJavaClass(outDirName, fileName)
{
def className = fileName.substring(0,fileName.size()-5)
outputFileName = outDirName.toString() + File.separator + fileName
outputFile = new File(outputFileName)
outputFile.write "package ${packageName};${NEW_LINE.multiply(2)}"
outputFile << "public class ${className}${NEW_LINE}"
outputFile << "{${NEW_LINE}"
outputFile << "${SINGLE_INDENT}public static void main(final String[] arguments)"
outputFile << "${NEW_LINE}${SINGLE_INDENT}{${NEW_LINE}"
outputFile << DOUBLE_INDENT << 'final String someString = "Dustin";' << NEW_LINE
outputFile << "${SINGLE_INDENT}}${NEW_LINE}"
outputFile << buildManyMethods()
outputFile << "}"
return outputFileName
}


/**
* Compile the provided Java source code file name.
*
* @param fileName Name of Java file to be compiled.
*/
def void compileJavaClass(fileName)
{
// Use the Java SE 6 Compiler API (JSR 199)
// http://java.sun.com/mailers/techtips/corejava/2007/tt0307.html#1
compiler = ToolProvider.getSystemJavaCompiler()

// The use of nulls in the call to JavaCompiler.run indicate use of defaults
// of System.in, System.out, and System.err.
int compilationResult = compiler.run(null, null, null, fileName)
if (compilationResult == 0)
{
println "${fileName} compiled successfully"
}
else
{
println "${fileName} compilation failed"
}
}


/**
* Create directories to which generated files will be written.
*
* @param baseDir The base directory used in which subdirectories for Java
* source packages will be generated.
*/
def String createDirectories(baseDir)
{
def outDirName = new StringBuilder(baseDir)
for (pkgDir in packagePieces)
{
outDirName << File.separator << pkgDir
}
outputDirectory = new File(outDirName.toString())
if (outputDirectory.exists() && outputDirectory.isDirectory())
{
println "Directory ${outDirName} already exists."
}
else
{
isDirectoryCreated = outputDirectory.mkdirs() // Use mkdirs in case multiple
println "Directory ${outputDirectoryName} ${isDirectoryCreated ? 'is' : 'not'} created."
}
return outDirName.toString()
}


/**
* Generate the body of generated Java class source code's main function.
*/
def String buildManyMethods()
{
def str = new StringBuilder() << NEW_LINE
for (i in 0..numberOfMethods)
{
str << SINGLE_INDENT << "private void doMethod${i}(){}" << NEW_LINE
}
return str
}

When the above script is run with a parameter of 5 for the number of methods, the following Java code is generated.


package dustin.examples;

public class LotsOfMethods
{
public static void main(final String[] arguments)
{
final String someString = "Dustin";
}

private void doMethod0(){}
private void doMethod1(){}
private void doMethod2(){}
private void doMethod3(){}
private void doMethod4(){}
private void doMethod5(){}
}

When I turn up the number of generated methods to 65000 methods, I run out of heap space as shown in the next screen snapshot.

The next screen snapshot shows the output of running the script again, but this time with 512 MB maximum heap space specified for the JVM.

What happens when we try to compile a class with too many methods? That is shown in the next screen snapshot that demonstrates what happens when just such a compilation is attempted.

The “too many constants” error message is shown with a pointer at the class keyword in the class definition. The method has too many methods to compile.

When I run javap -c -private dustin.examples.LotsOfMethods (-c to disassemble the code, -private to display the many private methods, and dustin.examples.LotsOfMethods is the name of the generated Java class), I see output like the following (only the first and end shown instead of displaying all 60,000+ methods).


Compiled from "LotsOfMethods.java"
public class dustin.examples.LotsOfMethods extends java.lang.Object{
public dustin.examples.LotsOfMethods();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: return

public static void main(java.lang.String[]);
Code:
0: return

private void doMethod0();
Code:
0: return

private void doMethod1();
Code:
0: return

private void doMethod2();
Code:
0: return

private void doMethod3();
Code:
0: return

private void doMethod4();
Code:
0: return

private void doMethod5();
Code:
0: return

private void doMethod6();
Code:
0: return

private void doMethod7();
Code:
0: return

private void doMethod8();
Code:
0: return

. . .

. . .

. . .

private void doMethod64992();
Code:
0: return

private void doMethod64993();
Code:
0: return

private void doMethod64994();
Code:
0: return

private void doMethod64995();
Code:
0: return

private void doMethod64996();
Code:
0: return

private void doMethod64997();
Code:
0: return

private void doMethod64998();
Code:
0: return

private void doMethod64999();
Code:
0: return

private void doMethod65000();
Code:
0: return

}

Conclusion

As with the last blog post, this post used Groovy and the Java Compiler API to intentionally reproduce an error that we hope to not see very often.

Additional Reference

Error Writing File: too many constants

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