Spring Utility Classes Overview: Assertions, ObjectUtils, CollectionUtils, File/IO, and Reflection/AOP

Introduction: The author, a senior architect, noticed many duplicate utility classes in colleagues' code and realized Spring already provides many of these functions, so they compiled this guide.

01 Assertion

Assertions are logical checks for impossible situations.

Assert keyword introduced in JDK 1.4, enabled via -enableassertions .

SpringBoot provides Assert utility class for data validation.

void notNull(Object object, String message)
void isNull(Object object, String message)
void isTrue(boolean expression, String message)
void notEmpty(Collection collection, String message)
void hasLength(String text, String message)
void hasText(String text, String message)
void isInstanceOf(Class type, Object obj, String message)
void isAssignable(Class superType, Class subType, String message)

02 Object, Array, Collection

ObjectUtils methods for class name, hash code, toString, etc.

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

CollectionUtils methods for checking emptiness, containment, uniqueness, merging collections, etc.

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 findValueOfType(Collection
collection, Class
type)

03 File, Resource, IO Stream

FileCopyUtils methods for copying between files, streams, and byte arrays.

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)

ResourceUtils methods for URL handling and file retrieval.

static boolean isUrl(String resourceLocation)
static URL getURL(String resourceLocation)
static File getFile(String resourceLocation)

StreamUtils methods for stream copying and draining.

void copy(byte[] in, OutputStream out)
int copy(InputStream in, OutputStream out)
void copy(byte[] in, OutputStream out)
int copyRange(InputStream in, OutputStream out, long start, long end)
int drain(InputStream in)

04 Reflection and AOP

ReflectionUtils methods for finding methods, fields, constructors, and invoking them.

Method findMethod(Class
clazz, String name)
Method findMethod(Class
clazz, String name, Class
... paramTypes)
Method[] getAllDeclaredMethods(Class
leafClass)
Constructor
accessibleConstructor(Class
clazz, Class
... parameterTypes)
Object invokeMethod(Method method, Object target, Object... args)
void makeAccessible(Method method)
Field findField(Class
clazz, String name)
Object getField(Field field, Object target)
void setField(Field field, Object target, Object value)

AopUtils methods for checking proxy types.

boolean isAopProxy()
boolean isJdkDynamicProxy()
boolean isCglibProxy()

AopContext to obtain the current proxy.

Object currentProxy()

The guide provides concise reference for developers to replace custom utility code with Spring’s built‑in helpers, improving code quality and reducing duplication in backend Java projects.