Master Spring’s Essential Utility Classes: From Resources to AOP

1. Resource Utility Class

The ResourceUtils class offers methods to resolve resource locations to files in the file system.

Read a file from the classpath

<code>File file = ResourceUtils.getFile(ResourceUtils.CLASSPATH_URL_PREFIX + "logback.xml");</code>

Read a file from the file system

<code>file = ResourceUtils.getFile(ResourceUtils.FILE_URL_PREFIX + "D:\\pom.xml");</code>

Supported prefixes

<code>public static final String CLASSPATH_URL_PREFIX = "classpath:";
public static final String FILE_URL_PREFIX = "file:";
public static final String JAR_URL_PREFIX = "jar:";
public static final String WAR_URL_PREFIX = "war:";</code>

2. Object Utility Class

Convert an object to an array

<code>Object obj = new int[] {1, 2, 3, 4};
Object[] arr = ObjectUtils.toObjectArray(obj);</code>

Add a new element to an array

<code>Integer[] obj = new Integer[] {1, 2, 3, 4};
Integer[] ret = ObjectUtils.addObjectToArray(obj, 5);</code>

Check if an element exists

<code>Integer[] obj = new Integer[] {1, 2, 3, 4};
boolean exists = ObjectUtils.containsElement(ret, 5);</code>

3. Array Conversion

The NumberUtils class provides convenient methods for number conversion and parsing.

Convert to a target type

<code>Number n = 10D;
NumberUtils.convertNumberToTargetClass(n, Double.class);</code>

Parse a number

<code>Long ret = NumberUtils.parseNumber("10", Long.class);
System.out.println(ret);</code>

4. Stream Operations

Copy a file stream into memory

<code>FileInputStream fis = new FileInputStream(new File("d:\\1.txt"));
ByteArrayOutputStream baos = new ByteArrayOutputStream();
// File content is now in baos
StreamUtils.copy(fis, baos);</code>

Read text content into memory

<code>StreamUtils.copy("abcdef", Charset.forName("UTF-8"), baos);</code>

Convert a file stream directly to a String

<code>FileInputStream fis = new FileInputStream(new File("d:\\1.txt"));
String content = StreamUtils.copyToString(fis, Charset.forName("UTF-8"));</code>

5. System Property Parsing

Resolve system property placeholders

<code>String home = SystemPropertyUtils.resolvePlaceholders("${java.home}");
System.out.println(home);</code>

6. Collection Utility Class

(Illustrated with an image in the original article.)

7. AOP Proxy Utilities

Check if an object is a proxy

<code>static class UserService {}
public static void main(String[] args) {
    ProxyFactory factory = new ProxyFactory();
    factory.addAdvice(new MethodInterceptor() {
        @Override
        public Object invoke(MethodInvocation invocation) throws Throwable {
            return invocation.proceed();
        }
    });
    factory.setTarget(new UserService());
    Object proxy = factory.getProxy();
    System.out.println(AopUtils.isAopProxy(proxy));
}</code>

Determine if a pointcut can apply to a target class

<code>@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
static @interface Pack {}
static class UserService {
    @Pack
    public void save() {}
}
// Custom pointcut
Pointcut pc = new Pointcut() {
    @Override
    public MethodMatcher getMethodMatcher() {
        return new AnnotationMethodMatcher(Pack.class);
    }
    @Override
    public ClassFilter getClassFilter() {
        return ClassFilter.TRUE;
    }
};
boolean ret = AopUtils.canApply(pc, UserService.class);
System.out.println(ret);
</code>

Obtain the original target of a proxy

<code>Object proxy = factory.getProxy();
AopProxyUtils.getSingletonTarget(proxy);
</code>

8. BeanFactory Utility

Provides convenient methods for operating on a BeanFactory , especially on ListableBeanFactory .

<code>AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext("com.pack");
BeanFactoryUtils.beanOfTypeIncludingAncestors(context, UserDAO.class);
</code>

9. Annotation Utility

The AnnotatedElementUtils class offers rich methods to find annotations, meta‑annotations, and repeatable annotations on elements such as methods, fields, or classes.

<code>AnnotatedElementUtils.hasAnnotation(UserService.class, Pack.class);
</code>

10. Bean Utility

The BeanUtils class supports bean instantiation, property type checking, property copying, and method discovery.

<code>// Instantiate an object
Person person = BeanUtils.instantiateClass(Person.class);
Person target = new Person();
// Copy properties
BeanUtils.copyProperties(person, target);
// Find a method
Method m = BeanUtils.findMethod(Person.class, "getName");
// Get property descriptor from method
PropertyDescriptor pd = BeanUtils.findPropertyForMethod(Person.class.getDeclaredMethod("getName"));
System.out.println(pd.getName());
</code>

11. Data Validation

Spring MVC validation can be performed via annotations or programmatically using ValidationUtils with custom validators.

<code>static class Person {
    private Integer age;
    private String name;
}
static class PersonValidator implements Validator {
    @Override
    public boolean supports(Class<?> clazz) { return Person.class.isAssignableFrom(clazz); }
    @Override
    public void validate(Object target, Errors errors) {
        Person p = (Person) target;
        if (p.age == null) {
            errors.reject("age.empty", "Age cannot be null");
        }
    }
}
public static void main(String[] args) {
    Validator validator = new PersonValidator();
    Person target = new Person();
    BeanPropertyBindingResult errors = new BeanPropertyBindingResult(target, "person");
    ValidationUtils.invokeValidator(validator, target, errors);
    System.out.println(errors);
}
</code>

12. XML Parsing

The DomUtils class simplifies DOM manipulation.

<code>DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document document = builder.parse(new ClassPathResource("com/pack/main/utils/person.xml").getInputStream());
Element element = document.getDocumentElement();
List<Element> elements = DomUtils.getChildElements(element);
elements.forEach(elt -> System.out.println(elt.getTagName() + "=" + elt.getFirstChild().getNodeValue()));
</code>

13. HTML Conversion

The HtmlUtils class provides HTML escaping based on the W3C HTML 4.01 recommendation.

<code>String ret = HtmlUtils.htmlEscape("<script>alert('script')</script>");
System.out.println(ret); // &lt;script&gt;alert(&#39;script&#39;)&lt;/script&gt;
</code>

14. Web Utilities

The WebUtils class offers a collection of helpful web‑related methods.

15. URI Handling

Utility methods for working with URIs are provided (illustrated with an image in the original article).

All the above content constitutes the complete article and is intended to help developers work more efficiently with Spring’s utility classes.