JVM学习（四）JVM中的对象及引用，2024年最新javalinux面试题

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正文

对象的内存布局大体如下：

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不一定

如果是数组

对象

对象头

实例数据

对齐填充

对象类型指针

数组长度

mark word

哈希码

对象分代年龄

锁状态标识

线程持有的锁

偏向线程id

偏向时间戳

在一个对象中，一定会有的是对象头和实例数据，但是不一定会有对齐填充。因为虚拟机规范中规范每个对象的大小都必须是8byte的整数倍，如果当前对象的实例数据+对象头的大小不是8字节的整数倍，那就需要对齐填充，将这个对象填充够8字节的整数倍。

整理一下对象的内存布局

1. 对象头

2. 对象运行时数据（mark word）

3. 哈希码

4. GC分代年龄

5. 锁状态标识

6. 线程持有的锁

7. 偏向线程id

8. 偏向时间戳

9. 对象的类型指针

10. 如果对象是数组类型，那还会存储数组的长度

11. 实例数据

12. 对齐填充（不是100%有）

对象的访问定位

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对象访问定位有两种方式：

句柄池

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使用句柄池的方式访问对象的意思是，栈帧中局部变量中的引用指向的句柄池中的某个句柄，然后这个句柄再指向堆中的某个对象，大概就是这样：

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局部变量表中的引用

句柄A

对象1

优点：这样做的优点是局部变量表中的引用是指向句柄池中的句柄的，当对象发生变化（被回收或被移动到其他的内存地址）时，只需要更新句柄池对对象的引用即可，这样可以保证程序的稳定

缺点：由于引入了句柄池，导致了对象的访问多了一次处理从而降低了访问速度

引申：句柄池在堆里

直接指针

---

直接指针就是这样的：中间没有句柄池，局部变量表中的引用直接指向堆中的内存对象

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局部变量表中的引用

对象1

优点：没有句柄池，也就没有中间过程，对象的访问速度提升了

缺点：实现复杂

引申：现在的Hotspot是使用直接指针的方式访问对象的

垃圾回收

====================================================================

众所周知，JVM的内存是自动回收的，开发人员大多数情况下不需要手动申请（malloc）、释放（free）内存。但是像C、C++这类语言是需要手动申请、释放内存的。

手动管理内存的申请释放容易产生两种问题：

1. 忘记释放

会导致内存泄露

2. 反复释放

会导致释放有用的内存对象，比如下面这样：

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线程1

malloc

do something

free

do something

free

线程2

do something

do something

malloc

do something

线程1 M   F   F

线程2        M

也就是说线程1free了内存之后，线程2malloc了相同地址的内存，然后线程1反复释放了同一个地址的内存，这就会导致线程2在相同地址上创建的对象丢失

如何判断对象的是否是垃圾

---

引用计数法

在引用计数法中，如果没有任何引用指向一个或多个对象，那么这些对象就是垃圾的

对象上记录了自己被引用的次数，每次引用会使引用次数+1，每次置空会使引用-1

存在的问题：引用计数法无法解决循环引用的问题

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引用

引用

对象1

对象2

上面这种引用关系下，对象1的引用计数为1，对象2的引用计数也是1，虽然它们与整体已经没有任何关系，但是因为彼此之间持有引用，就导致引用计数!=0最终无法被回收

可达性分析算法

JVM采用的是这种判断对象是否为垃圾的方式，JVM从GC roots开始查找对象的引用链，凡是在引用链上的对象都是活对象，其他对象为垃圾对象

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GC roots

静态变量

常量

JNI

局部变量表中引用的对象

…

class的回收机制

---

class的回收机制非常严格，需要同时满足以下条件才能回收

1. 堆中已经不存在当前class的任何实例

2. classLoader已经被回收

3. 对Class对象没有任何引用

所以说，class几乎不可能被回收，可以使用参数-Xnoclassgc来禁用class回收，这样可以稍微提高gc效率

总结

机会是留给有准备的人，大家在求职之前应该要明确自己的态度，熟悉求职流程，做好充分的准备，把一些可预见的事情做好。

对于应届毕业生来说，校招更适合你们，因为绝大部分都不会有工作经验，企业也不会有工作经验的需求。同时，你也不需要伪造高大上的实战经验，以此让自己的简历能够脱颖而出，反倒会让面试官有所怀疑。

你在大学时期应该明确自己的发展方向，如果你在大一就确定你以后想成为Java工程师，那就不要花太多的时间去学习其他的技术语言，高数之类的，不如好好想着如何夯实Java基础。下图涵盖了应届生乃至转行过来的小白要学习的Java内容：

请转发本文支持一下

网上学习资料一大堆，但如果学到的知识不成体系，遇到问题时只是浅尝辄止，不再深入研究，那么很难做到真正的技术提升。

需要这份系统化的资料的朋友，可以添加V获取：vip1024b （备注Java）

一个人可以走的很快，但一群人才能走的更远！不论你是正从事IT行业的老鸟或是对IT行业感兴趣的新人，都欢迎加入我们的的圈子（技术交流、学习资源、职场吐槽、大厂内推、面试辅导），让我们一起学习成长！
基础。下图涵盖了应届生乃至转行过来的小白要学习的Java内容：

请转发本文支持一下

[外链图片转存中…(img-ly2VSsyr-1713422221639)]

[外链图片转存中…(img-ByBzjkXu-1713422221640)]

网上学习资料一大堆，但如果学到的知识不成体系，遇到问题时只是浅尝辄止，不再深入研究，那么很难做到真正的技术提升。

需要这份系统化的资料的朋友，可以添加V获取：vip1024b （备注Java）
[外链图片转存中…(img-7WYCNGrz-1713422221640)]

一个人可以走的很快，但一群人才能走的更远！不论你是正从事IT行业的老鸟或是对IT行业感兴趣的新人，都欢迎加入我们的的圈子（技术交流、学习资源、职场吐槽、大厂内推、面试辅导），让我们一起学习成长！