Java 中canonical name, simple name and class name 的区别

转发：

请先了解这个程序：

class ClassNameTest {public static void main(final String... arguments) {printNamesForClass(int.class,"int.class (primitive)");printNamesForClass(String.class,"String.class (ordinary class)");printNamesForClass(java.util.HashMap.SimpleEntry.class,"java.util.HashMap.SimpleEntry.class (nested class)");printNamesForClass(new java.io.Serializable(){}.getClass(),"new java.io.Serializable(){}.getClass() (anonymous inner class)");}private static void printNamesForClass(final Class<?> clazz, final String label) {System.out.println(label + ":");System.out.println("    getName():          " + clazz.getName());System.out.println("    getCanonicalName(): " + clazz.getCanonicalName());System.out.println("    getSimpleName():    " + clazz.getSimpleName());System.out.println("    getTypeName():      " + clazz.getTypeName()); // added in Java 8System.out.println();}
}

Output：

int.class (primitive):getName():          intgetCanonicalName(): intgetSimpleName():    intgetTypeName():      intString.class (ordinary class):getName():          java.lang.StringgetCanonicalName(): java.lang.StringgetSimpleName():    StringgetTypeName():      java.lang.Stringjava.util.HashMap.SimpleEntry.class (nested class):getName():          java.util.AbstractMap$SimpleEntrygetCanonicalName(): java.util.AbstractMap.SimpleEntrygetSimpleName():    SimpleEntrygetTypeName():      java.util.AbstractMap$SimpleEntrynew java.io.Serializable(){}.getClass() (anonymous inner class):getName():          ClassNameTest$1getCanonicalName(): nullgetSimpleName():    getTypeName():      ClassNameTest$1

There's an empty entry in the last block where getSimpleName returns an empty string.

The upshot looking at this is:

the name is the name that you'd use to dynamically load the class with, for example, a call to Class.forName with the default ClassLoader. Within the scope of a certain ClassLoader, all classes have unique names.

the canonical name is the name that would be used in an import statement. It might be useful during toString or logging operations. When the javac compiler has complete view of a classpath, it enforces uniqueness of canonical names within it by clashing fully qualified class and package names at compile time. However JVMs must accept such name clashes, and thus canonical names do not uniquely identify classes within a ClassLoader. (In hindsight, a better name for this getter would have been getJavaName; but this method dates from a time when the JVM was used solely to run Java programs.)

the simple name loosely identifies the class, again might be useful during toString or logging operations but is not guaranteed to be unique.

the type name returns "an informative string for the name of this type", "It's like toString: it's purely informative and has no contract value". (as written by sir4ur0n)

Also you can commonly reference the Java Language Specification documentation for these types technical Java API details:

Here's the Java 11 Specification on this subject matter: Chapter 6. Names

Example 6.7-2. and Example 6.7-2. goes over Fully Qualified Names and Fully Qualified Names v. Canonical Name respectively

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