Spring Boot Utility Classes Overview: Assertions, ObjectUtils, StringUtils, CollectionUtils, FileCopyUtils, ReflectionUtils, and AopUtils

Recently the author noticed many duplicated utility classes in a project and discovered that Spring already provides a rich set of utilities. The following sections summarize the most useful Spring Boot helper classes with code snippets.

1. Assertions

Assertions are logical checks to ensure that illegal states never occur. Spring's Assert class offers methods such as notNull(Object object, String message) , isNull(Object object, String message) , isTrue(boolean expression, String message) , notEmpty(Collection collection, String message) , hasLength(String text, String message) , hasText(String text, String message) , isInstanceOf(Class type, Object obj, String message) , and isAssignable(Class superType, Class subType, String message) .

2. ObjectUtils

Utility methods for handling objects safely:

String nullSafeClassName(Object obj)
int nullSafeHashCode(Object object)
String nullSafeToString(boolean[] array)
String getIdentityHexString(Object obj)
String identityToString(Object obj)
String getDisplayString(Object obj)

3. StringUtils

String‑related checks and operations:

boolean isEmpty(Object str)
boolean endsWithIgnoreCase(String str, String suffix)
boolean startsWithIgnoreCase(String str, String prefix)
boolean containsWhitespace(String str)
boolean hasLength(CharSequence str)
boolean hasText(CharSequence str)
boolean substringMatch(CharSequence str, int index, CharSequence substring)
int countOccurrencesOf(String str, String sub)
String replace(String inString, String oldPattern, String newPattern)
String trimTrailingCharacter(String str, char trailingCharacter)
String trimLeadingCharacter(String str, char leadingCharacter)
String trimLeadingWhitespace(String str)
String trimTrailingWhitespace(String str)
String trimWhitespace(String str)
String trimAllWhitespace(String str)
String delete(String inString, String pattern)
String deleteAny(String inString, String charsToDelete)
String[] trimArrayElements(String[] array)
String uriDecode(String source, Charset charset)

4. CollectionUtils

Helpers for collections:

boolean isEmpty(Collection
collection)
boolean isEmpty(Map
map)
boolean containsInstance(Collection
collection, Object element)
boolean contains(Iterator
iterator, Object element)
boolean containsAny(Collection
source, Collection
candidates)
boolean hasUniqueObject(Collection
collection)
void mergeArrayIntoCollection(Object array, Collection
collection)
void mergePropertiesIntoMap(Properties props, Map
map)
T lastElement(List
list)
T lastElement(Set
set)
E findFirstMatch(Collection
source, Collection
candidates)
T findValueOfType(Collection
collection, Class
type)
Object findValueOfType(Collection
collection, Class
[] types)
Class
findCommonElementType(Collection
collection)

5. FileCopyUtils

File and stream copy utilities:

byte[] copyToByteArray(File in)
byte[] copyToByteArray(InputStream in)
String copyToString(Reader in)
void copy(byte[] in, File out)
int copy(File in, File out)
void copy(byte[] in, OutputStream out)
int copy(InputStream in, OutputStream out)
int copy(Reader in, Writer out)
void copy(String in, Writer out)

6. ResourceUtils & Resource

Resource handling helpers:

boolean isUrl(String resourceLocation)
URL getURL(String resourceLocation)
File getFile(String resourceLocation)
boolean exists()
File getFile()
URI getURI()
URL getURL()
InputStream getInputStream()
String getDescription()

7. StreamUtils

Stream copy helpers:

void copy(byte[] in, OutputStream out)
int copy(InputStream in, OutputStream out)
void copy(String in, Charset charset, OutputStream out)
long copyRange(InputStream in, OutputStream out, long start, long end)
byte[] copyToByteArray(InputStream in)
String copyToString(InputStream in, Charset charset)
int drain(InputStream in)

8. ReflectionUtils

Reflection helpers for methods, fields and constructors:

Method findMethod(Class
clazz, String name)
Method findMethod(Class
clazz, String name, Class
... paramTypes)
Method[] getAllDeclaredMethods(Class
leafClass)
Constructor
accessibleConstructor(Class
clazz, Class
... parameterTypes)
boolean isEqualsMethod(Method method)
boolean isHashCodeMethod(Method method)
boolean isToStringMethod(Method method)
boolean isObjectMethod(Method method)
boolean declaresException(Method method, Class
exceptionType)
Object invokeMethod(Method method, Object target)
Object invokeMethod(Method method, Object target, Object... args)
void makeAccessible(Method method)
void makeAccessible(Constructor
ctor)
Field findField(Class
clazz, String name)
Field findField(Class
clazz, String name, Class
type)
boolean isPublicStaticFinal(Field field)
Object getField(Field field, Object target)
void setField(Field field, Object target, Object value)
void shallowCopyFieldState(Object src, Object dest)
void makeAccessible(Field field)
void doWithFields(Class
clazz, ReflectionUtils.FieldCallback fc)
void doWithFields(Class
clazz, ReflectionUtils.FieldCallback fc, ReflectionUtils.FieldFilter ff)
void doWithLocalFields(Class
clazz, ReflectionUtils.FieldCallback fc)

9. AopUtils & AopContext

AOP helper utilities:

boolean isAopProxy()
boolean isJdkDynamicProxy()
boolean isCglibProxy()
Class
getTargetClass()
Object currentProxy()

The article concludes by inviting readers to discuss, ask questions, and join the author’s community for further knowledge sharing.