import语句只在编译时有效

其实import了之后没用的话跟JVM啊classloader什么的完全没关系。
import语句只在编译时有效，编译过后不会带到class文件里的。所以import了之后用了的类的信息才会出现在编译出来的class文件里，没用的则不会出现。

有兴趣可以自己动手试试。

//import java.util.regex.*;

public class Test {
    public static void main(String[] args) {
        System.out.println("test");
    }
}

//import java.util.regex.*;

public class Test {
    public static void main(String[] args) {
        System.out.println("test");
    }
}

那行import无论被注释掉还是没有被注释掉，编译出来的class文件的内容都是这样的：（javap -private -s -verbose Test）

Compiled from "Test.java"
public class Test extends java.lang.Object
  SourceFile: "Test.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method   #6.#15; //  java/lang/Object."<init>":()V
const #2 = Field    #16.#17;    //  java/lang/System.out:Ljava/io/PrintStream;
const #3 = String   #18;    //  test
const #4 = Method   #19.#20;    //  java/io/PrintStream.println:(Ljava/lang/String;)V
const #5 = class    #21;    //  Test
const #6 = class    #22;    //  java/lang/Object
const #7 = Asciz    <init>;
const #8 = Asciz    ()V;
const #9 = Asciz    Code;
const #10 = Asciz   LineNumberTable;
const #11 = Asciz   main;
const #12 = Asciz   ([Ljava/lang/String;)V;
const #13 = Asciz   SourceFile;
const #14 = Asciz   Test.java;
const #15 = NameAndType #7:#8;//  "<init>":()V
const #16 = class   #23;    //  java/lang/System
const #17 = NameAndType #24:#25;//  out:Ljava/io/PrintStream;
const #18 = Asciz   test;
const #19 = class   #26;    //  java/io/PrintStream
const #20 = NameAndType #27:#28;//  println:(Ljava/lang/String;)V
const #21 = Asciz   Test;
const #22 = Asciz   java/lang/Object;
const #23 = Asciz   java/lang/System;
const #24 = Asciz   out;
const #25 = Asciz   Ljava/io/PrintStream;;
const #26 = Asciz   java/io/PrintStream;
const #27 = Asciz   println;
const #28 = Asciz   (Ljava/lang/String;)V;

{
public Test();
  Signature: ()V
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 3: 0


public static void main(java.lang.String[]);
  Signature: ([Ljava/lang/String;)V
  Code:
   Stack=2, Locals=1, Args_size=1
   0:   getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc #3; //String test
   5:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return
  LineNumberTable:
   line 5: 0
   line 6: 8


}

Compiled from "Test.java"
public class Test extends java.lang.Object
  SourceFile: "Test.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method	#6.#15;	//  java/lang/Object."<init>":()V
const #2 = Field	#16.#17;	//  java/lang/System.out:Ljava/io/PrintStream;
const #3 = String	#18;	//  test
const #4 = Method	#19.#20;	//  java/io/PrintStream.println:(Ljava/lang/String;)V
const #5 = class	#21;	//  Test
const #6 = class	#22;	//  java/lang/Object
const #7 = Asciz	<init>;
const #8 = Asciz	()V;
const #9 = Asciz	Code;
const #10 = Asciz	LineNumberTable;
const #11 = Asciz	main;
const #12 = Asciz	([Ljava/lang/String;)V;
const #13 = Asciz	SourceFile;
const #14 = Asciz	Test.java;
const #15 = NameAndType	#7:#8;//  "<init>":()V
const #16 = class	#23;	//  java/lang/System
const #17 = NameAndType	#24:#25;//  out:Ljava/io/PrintStream;
const #18 = Asciz	test;
const #19 = class	#26;	//  java/io/PrintStream
const #20 = NameAndType	#27:#28;//  println:(Ljava/lang/String;)V
const #21 = Asciz	Test;
const #22 = Asciz	java/lang/Object;
const #23 = Asciz	java/lang/System;
const #24 = Asciz	out;
const #25 = Asciz	Ljava/io/PrintStream;;
const #26 = Asciz	java/io/PrintStream;
const #27 = Asciz	println;
const #28 = Asciz	(Ljava/lang/String;)V;

{
public Test();
  Signature: ()V
  Code:
   Stack=1, Locals=1, Args_size=1
   0:	aload_0
   1:	invokespecial	#1; //Method java/lang/Object."<init>":()V
   4:	return
  LineNumberTable: 
   line 3: 0


public static void main(java.lang.String[]);
  Signature: ([Ljava/lang/String;)V
  Code:
   Stack=2, Locals=1, Args_size=1
   0:	getstatic	#2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:	ldc	#3; //String test
   5:	invokevirtual	#4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:	return
  LineNumberTable: 
   line 5: 0
   line 6: 8


}

所以别在意，import多余的东西不会对运行性能带来影响。JVM根本不知道你import了哪些类，只知道你用了哪些类。

import多余的东西主要带来的影响是污染了当前编译单元（也就是单个Java源代码文件）的命名空间而已，名字冲突的可能性变大了而已。例如说package A里有MyClass，你当前的文件在package B里，也有MyClass，这个时候直接写MyClass就容易有歧义，需要写成完整的A.MyClass或B.MyClass来区分。只是这个问题而已。