遍歷并批量刪除容器中元素出現(xiàn)ConcurrentModificationException...

WindySky 2008-09-28

展開全文

在以下四種遍歷過程中,前兩種會拋出ConcurrentModificationException,而后兩種方法是正確的.

Department類:

package com.sitinspring;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Department {
    private String name;

    private List<Member> memberSheet;

    public Department(String name) {
        this.name = name;
    }

    public void addMemer(Member member) {
        if (memberSheet == null) {
            memberSheet = new ArrayList<Member>();
        }

        memberSheet.add(member);
    }

    public void printMemberSheet() {
        System.out.println("----部門" + name + "人員名單---");

        for (Member member : memberSheet) {
            System.out.println(member);
        }
    }

    /**
     * 里面的四個清除過程請分別獨立執(zhí)行
     *
     */
    public void removeYoungerFromMemberSheet() {
        // 遍歷一:這個處理會拋出java.util.ConcurrentModificationException
        for (Member member : memberSheet) {
            if (member.getAge() < 30) {
                memberSheet.remove(member);
            }
        }

        // 遍歷二:這個處理也會拋出java.util.ConcurrentModificationException
        for (Iterator it = memberSheet.iterator(); it.hasNext();) {
            Member member = (Member) it.next();

            if (member.getAge() < 30) {
                memberSheet.remove(member);
            }
        }

        // 遍歷三:這個處理調(diào)用Iterator 本身的方法 remove(),會正常執(zhí)行
        for (Iterator it = memberSheet.iterator(); it.hasNext();) {
            Member member = (Member) it.next();

            if (member.getAge() < 30) {
                it.remove();
            }
        }

        // 遍歷四:這個處理不依賴Iterator,也會正常執(zhí)行
        for (int i=0;i<memberSheet.size();i++) {
            Member member = memberSheet.get(i);

            if (member.getAge() < 30) {
                memberSheet.remove(member);
            }
        }
    }

    public String toString() {
        return name;
    }

    public String getName() {
        return name;
    }

    public static void main(String[] args) {
        Department resarchDept = new Department("研發(fā)部門");
        resarchDept.addMemer(new Member("張三", 38));
        resarchDept.addMemer(new Member("李四", 24));
        resarchDept.addMemer(new Member("王五", 30));
        resarchDept.addMemer(new Member("錢七", 22));
        resarchDept.addMemer(new Member("孫八", 39));
        resarchDept.addMemer(new Member("周九", 30));

        resarchDept.removeYoungerFromMemberSheet();
        resarchDept.printMemberSheet();
    }
}

Member類:

package com.sitinspring;

public class Member{
    private String name;
    private int age;
    private Department department;

    public Member(String name,int age){
        this.name=name;
        this.age=age;
    }

    public String toString(){
        StringBuffer sb=new StringBuffer();
        sb.append("員工名="+name);
        sb.append(" 年齡="+age);

        if(department!=null){
            sb.append(" 所屬部門="+department);
        }

        return sb.toString();
    }

    public void changeNewDepartment(Department newDepartment) {
        department.removeMemer(name);
        newDepartment.addMemer(this);
    }

    public String getName() {
        return name;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }
}

為什么會發(fā)生這樣的結(jié)果呢?這是因為

"當(dāng)使用 fail-fast iterator 對 Collection 或 Map 進(jìn)行迭代操作過程中嘗試直接修改 Collection / Map 的內(nèi)容時，即使是在單線程下運行， java.util.ConcurrentModificationException 異常也將被拋出。
Iterator 是工作在一個獨立的線程中，并且擁有一個 mutex 鎖。 Iterator 被創(chuàng)建之后會建立一個指向原來對象的單鏈索引表，當(dāng)原來的對象數(shù)量發(fā)生變化時，這個索引表的內(nèi)容不會同步改變，所以當(dāng)索引指針往后移動的時候就找不到要迭代的對象，所以按照 fail-fast 原則 Iterator 會馬上拋出 java.util.ConcurrentModificationException 異常。
所以 Iterator 在工作的時候是不允許被迭代的對象被改變的。但你可以使用 Iterator 本身的方法 remove() 來刪除對象， Iterator.remove() 方法會在刪除當(dāng)前迭代對象的同時維護索引的一致性。"

上述這段資料來自http://hi.baidu.com/xjenator/blog/item/23b235a89041d4b0ca130c16.html.

java.util包中很多迭代器都是所謂的fail-fast迭代器.這些迭代器如果發(fā)現(xiàn)集合被修改,而且不是通過迭代器本身,那么拋出一個異常進(jìn)行清除-ConcurrentModificationException-從而避免不安全行為的發(fā)生.

因此,第三種采用it.remove();不會出現(xiàn)任何異常,而第四不依賴于Iterator而依賴于索引當(dāng)然更不會出現(xiàn)異常.

代碼下載:
http://www./Files/sitinspring/ConcurrentModificationTest20071203210937.rar