HideUtilityClassConstructorCheck.java
////////////////////////////////////////////////////////////////////////////////
// checkstyle: Checks Java source code for adherence to a set of rules.
// Copyright (C) 2001-2021 the original author or authors.
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
////////////////////////////////////////////////////////////////////////////////
package com.puppycrawl.tools.checkstyle.checks.design;
import com.puppycrawl.tools.checkstyle.StatelessCheck;
import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
import com.puppycrawl.tools.checkstyle.api.DetailAST;
import com.puppycrawl.tools.checkstyle.api.TokenTypes;
/**
* <p>
* Makes sure that utility classes (classes that contain only static methods or fields in their API)
* do not have a public constructor.
* </p>
* <p>
* Rationale: Instantiating utility classes does not make sense.
* Hence the constructors should either be private or (if you want to allow subclassing) protected.
* A common mistake is forgetting to hide the default constructor.
* </p>
* <p>
* If you make the constructor protected you may want to consider the following constructor
* implementation technique to disallow instantiating subclasses:
* </p>
* <pre>
* public class StringUtils // not final to allow subclassing
* {
* protected StringUtils() {
* // prevents calls from subclass
* throw new UnsupportedOperationException();
* }
*
* public static int count(char c, String s) {
* // ...
* }
* }
* </pre>
* <p>
* To configure the check:
* </p>
* <pre>
* <module name="HideUtilityClassConstructor"/>
* </pre>
* <p>
* Example:
* </p>
* <pre>
* class Test { // violation, class only has a static method and a constructor
*
* public Test() {
* }
*
* public static void fun() {
* }
* }
*
* class Foo { // OK
*
* private Foo() {
* }
*
* static int n;
* }
*
* class Bar { // OK
*
* protected Bar() {
* // prevents calls from subclass
* throw new UnsupportedOperationException();
* }
* }
*
* class UtilityClass { // violation, class only has a static field
*
* static float f;
* }
* </pre>
* <p>
* Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
* </p>
* <p>
* Violation Message Keys:
* </p>
* <ul>
* <li>
* {@code hide.utility.class}
* </li>
* </ul>
*
* @since 3.1
*/
@StatelessCheck
public class HideUtilityClassConstructorCheck extends AbstractCheck {
/**
* A key is pointing to the warning message text in "messages.properties"
* file.
*/
public static final String MSG_KEY = "hide.utility.class";
@Override
public int[] getDefaultTokens() {
return getRequiredTokens();
}
@Override
public int[] getAcceptableTokens() {
return getRequiredTokens();
}
@Override
public int[] getRequiredTokens() {
return new int[] {TokenTypes.CLASS_DEF};
}
@Override
public void visitToken(DetailAST ast) {
// abstract class could not have private constructor
if (!isAbstract(ast)) {
final boolean hasStaticModifier = isStatic(ast);
final Details details = new Details(ast);
details.invoke();
final boolean hasDefaultCtor = details.isHasDefaultCtor();
final boolean hasPublicCtor = details.isHasPublicCtor();
final boolean hasNonStaticMethodOrField = details.isHasNonStaticMethodOrField();
final boolean hasNonPrivateStaticMethodOrField =
details.isHasNonPrivateStaticMethodOrField();
final boolean hasAccessibleCtor = hasDefaultCtor || hasPublicCtor;
// figure out if class extends java.lang.object directly
// keep it simple for now and get a 99% solution
final boolean extendsJlo =
ast.findFirstToken(TokenTypes.EXTENDS_CLAUSE) == null;
final boolean isUtilClass = extendsJlo
&& !hasNonStaticMethodOrField && hasNonPrivateStaticMethodOrField;
if (isUtilClass && hasAccessibleCtor && !hasStaticModifier) {
log(ast, MSG_KEY);
}
}
}
/**
* Returns true if given class is abstract or false.
*
* @param ast class definition for check.
* @return true if a given class declared as abstract.
*/
private static boolean isAbstract(DetailAST ast) {
return ast.findFirstToken(TokenTypes.MODIFIERS)
.findFirstToken(TokenTypes.ABSTRACT) != null;
}
/**
* Returns true if given class is static or false.
*
* @param ast class definition for check.
* @return true if a given class declared as static.
*/
private static boolean isStatic(DetailAST ast) {
return ast.findFirstToken(TokenTypes.MODIFIERS)
.findFirstToken(TokenTypes.LITERAL_STATIC) != null;
}
/**
* Details of class that are required for validation.
*/
private static class Details {
/** Class ast. */
private final DetailAST ast;
/** Result of details gathering. */
private boolean hasNonStaticMethodOrField;
/** Result of details gathering. */
private boolean hasNonPrivateStaticMethodOrField;
/** Result of details gathering. */
private boolean hasDefaultCtor;
/** Result of details gathering. */
private boolean hasPublicCtor;
/**
* C-tor.
*
* @param ast class ast
* */
/* package */ Details(DetailAST ast) {
this.ast = ast;
}
/**
* Getter.
*
* @return boolean
*/
public boolean isHasNonStaticMethodOrField() {
return hasNonStaticMethodOrField;
}
/**
* Getter.
*
* @return boolean
*/
public boolean isHasNonPrivateStaticMethodOrField() {
return hasNonPrivateStaticMethodOrField;
}
/**
* Getter.
*
* @return boolean
*/
public boolean isHasDefaultCtor() {
return hasDefaultCtor;
}
/**
* Getter.
*
* @return boolean
*/
public boolean isHasPublicCtor() {
return hasPublicCtor;
}
/**
* Main method to gather statistics.
*/
public void invoke() {
final DetailAST objBlock = ast.findFirstToken(TokenTypes.OBJBLOCK);
hasNonStaticMethodOrField = false;
hasNonPrivateStaticMethodOrField = false;
hasDefaultCtor = true;
hasPublicCtor = false;
DetailAST child = objBlock.getFirstChild();
while (child != null) {
final int type = child.getType();
if (type == TokenTypes.METHOD_DEF
|| type == TokenTypes.VARIABLE_DEF) {
final DetailAST modifiers =
child.findFirstToken(TokenTypes.MODIFIERS);
final boolean isStatic =
modifiers.findFirstToken(TokenTypes.LITERAL_STATIC) != null;
if (isStatic) {
final boolean isPrivate =
modifiers.findFirstToken(TokenTypes.LITERAL_PRIVATE) != null;
if (!isPrivate) {
hasNonPrivateStaticMethodOrField = true;
}
}
else {
hasNonStaticMethodOrField = true;
}
}
if (type == TokenTypes.CTOR_DEF) {
hasDefaultCtor = false;
final DetailAST modifiers =
child.findFirstToken(TokenTypes.MODIFIERS);
if (modifiers.findFirstToken(TokenTypes.LITERAL_PRIVATE) == null
&& modifiers.findFirstToken(TokenTypes.LITERAL_PROTECTED) == null) {
// treat package visible as public
// for the purpose of this Check
hasPublicCtor = true;
}
}
child = child.getNextSibling();
}
}
}
}