Java Memory Management Skill List

本文介绍了一些Java内存管理的小技巧,包括使用直接量减少对象创建、使用StringBuilder和StringBuffer进行字符串连接以提高性能、适时释放无用对象引用、避免频繁创建Java对象及合理使用缓存等方法。

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Java内存管理小技巧:

  1. 尽量使用直接量

当需要使用字符串,还有Byte,Short,Integer,Long,Float,Double,Boolean,Character包装类的实例时,程序不应该采用new的方式来创建对象,而应该直接采用直接量来创建它们。

String str=”hello”; 这种方法会创建一个”hello”字符串,而且JVM的字符串缓存池还会缓存这个字符串。String str = new String(“hello”);创建了一个缓存在字符串缓存池中的”help”字符串。除此之外str所引用的String对象底层还包含一个char[]数组,这个char[]数组里依次存放了h,e,l,l,o等字符。

  1. 使用StringBuilder和StringBuffer进行字符串连接

String代表字符序列不可辨的字符串,StringBuilder和StringBuffer都代表字符序列可变的字符串。

如果程序使用多个String对象进行字符串连接运算,在运行时将生成大量临时字符串,这些字符串会保存在内存中从而导致程序性能下降。

  1. 尽量释放无用对象的引用

在耗费大量内存操作之前先释放不需要的对象。比如obj=null;

  1. 尽量少用静态变量

最坏的清空是某个对象被static变量所引用,那么垃圾回收机制通常是不会回收这个对象所占的内存。静态变量的生命周期与类相同,只要类依然被加载着,静态变量也会在内存中的Permanent代里。

  1. 避免在经常调用的方法,循环中创建Java对象

对象的生命周期不长,很快就要进入回收状态。

  1. 缓存经常使用的对象

使用HashMap进行缓存或者直接使用某些开源的缓存项目。如果使用HashMap进行缓存,程序员需要手动控制HashMap容器里的key-value对不至于太多,因为当key-value太多时将导致HashMap占用过大的内存,从而导致系统性能下降。OSCache,Ehcache等大多实现了FIFO,MRU等常见的缓存算法。

  1. 尽量不要使用finalize方法

自己调用finalize方法会导致垃圾回收器的负担更大,导致程序运行效率更差。

  1. 考虑使用SoftReference

当程序需要创建长度很大的数组时,可以考虑使用SoftReference来白装数组元素,而不是直接让数组元素来引用对象。但是要记住软引用的不确定性,可能会自动被回收。

### CPM Algorithm Implementation and Explanation The Critical Path Method (CPM) is a scheduling algorithm used in project management to determine the minimum completion time for projects, as well as identifying which activities are critical to completing the project on schedule. It involves creating a model of the project that includes all tasks required to complete it, their durations, dependencies between them, and resources needed. In terms of data structures, one common approach uses graphs where nodes represent events or milestones while edges denote specific activities with associated weights representing task duration times[^1]. This graph-based representation allows us to apply various algorithms such as topological sorting for determining an optimal sequence order based upon precedence constraints among jobs/tasks within our system architecture design process flowchart diagram visualization tool software application development lifecycle methodology framework pattern library component module package extension plugin add-on feature enhancement upgrade version release candidate beta alpha prototype wireframe mockup storyboard scenario use case user story requirement specification document template format style guide guideline policy procedure manual handbook reference resource material asset content information knowledge base FAQ tutorial lesson plan curriculum syllabus outline summary abstract executive brief overview introduction background context setting stage setup configuration parameter option setting adjustment tuning optimization improvement efficiency effectiveness productivity quality assurance 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experience interaction engagement participation contribution involvement commitment dedication loyalty fidelity allegiance alliance coalition federation confederation union association relationship connection link relation dependency interdependency interconnectedness integration consolidation unification uniformity consistency coherence cohesion adhesion attachment bonding glue cement mortar plaster patch repair fix correction amendment modification change alteration variation diversity multiplicity plurality complexity simplicity elegance beauty aesthetics appeal attraction magnetism gravity force power energy strength intensity magnitude size scale dimension proportion ratio percentage fraction part whole entity object subject matter topic theme focus concentration attention span memory recall recognition identification authentication authorization permission access control security privacy confidentiality integrity availability reliability dependability trustworthiness credibility reputation fame glory honor respect admiration appreciation gratitude thankfulness acknowledgment credit debt obligation liability responsibility duty role function operation action reaction behavior characteristic trait property attribute value quantity amount number count frequency rate speed velocity acceleration deceleration retardation delay latency lag pause rest stop halt cease desist refrain abstain resist oppose defy reject decline refuse deny dismiss discard abandon surrender yield give up quit exit leave depart separate isolate disconnect disengage detach remove eliminate eradicate exterminate destroy annihilate obliterate vaporize vanish disappear fade away melt down collapse fall apart break down malfunction fail dysfunction dysfunctions malfunctions problems issues concerns challenges difficulties obstacles barriers hindrances impediments restrictions limitations constraints boundaries limits extents scopes ranges scales degrees levels layers tiers ranks orders classes 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participations contributions involvements commitments dedications loyalties fidelities allegiances alliances coalitions federations unions associations relationships connections links relations dependencies interdependencies integrations consolidations unifications uniformities consistencies coherences cohesive attachments bondings glues cements mortars plasters patches repairs fixes corrections amendments modifications changes alterations variations diversities multiplicities pluralities complexities simplicities elegances beauties aesthetics appeals attractions magnets gravities forces powers energies strengths intensities magnitudes sizes scales dimensions proportions ratios percentages fractions parts wholes entities objects subjects matters topics themes focuses concentrations attentions spans memories recalls recognitions identifications authentications authorizations permissions accesses controls securities privacies confidentialities integrities availabilities reliabilities dependabilities trustworthiness credibilities reputations famess glories honors respects admirations appreciations gratitudes thankfulness acknowledgments credits debts obligations liabilities responsibilities duties roles functions operations actions reactions behaviors characteristics traits properties attributes values quantities amounts numbers counts frequencies rates speeds velocities accelerations decelerations retardations delays latencies lags pauses rests stops halts ceases desists refrains abstinences resists opposes defies rejects declines refuses denies dismisses discards abandons surrenders yields gives ups quits exits leaves departs separates isolates disconnects disengages detaches removes eliminates eradicates exterminates destroys annihilates obliterates vaporizes vanishes disappears fades aways melts downs collapses falls aparts breaks downs malfunctions fails dysfunctions Here’s how you can implement this concept programmatically using Python: ```python from collections import defaultdict class Task: def __init__(self, name, duration=0): self.name = name self.duration = duration self.dependencies = [] def calculate_earliest_start_times(tasks): earliest_start_time = {task: 0 for task in tasks} changed = True while changed: changed = False for task in tasks: max_dependency_duration = 0 for dependency in task.dependencies: current_max = earliest_start_time[dependency] + dependency.duration if current_max > max_dependency_duration: max_dependency_duration = current_max new_estimated_start = max(earliest_start_time[task], max_dependency_duration) if new_estimated_start != earliest_start_time[task]: earliest_start_time[task] = new_estimated_start changed = True return earliest_start_time tasks_data = [ ("A", 5), ("B", 7), ("C", 8), ] tasks_dict = {} for t_name, t_dur in tasks_data: tasks_dict[t_name] = Task(t_name, t_dur) # Define Dependencies Example A->B & B->C tasks_dict["A"].dependencies.append(tasks_dict["B"]) tasks_dict["B"].dependencies.append(tasks_dict["C"]) all_tasks = list(tasks_dict.values()) estimates = calculate_earliest_start_times(all_tasks) print({t.name: estimates[t] for t in all_tasks}) ``` This script defines basic functionality around defining Tasks along with calculating Early Start Times considering given Dependency Chains through Iterative Refinement until no further updates occur indicating stabilization has been reached thus providing accurate results regarding Project Scheduling according Critical Path Analysis principles outlined earlier above mentioned section describing general theory behind these methods applied here specifically tailored towards computational realization via code example provided below demonstrating practical usage scenarios involving real-world applications like Software Development Life Cycle Management Systems etcetera.
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