Transitioning for OOP Developers
In traditional object-oriented programming (OOP), most developers are accustomed to programming in the imperative/procedural style. To switch to developing in a pure functional style, they have to make a transition in their thinking and their approach to development.
To solve problems, OOP developers design class hierarchies, focus on proper encapsulation, and think in terms of class contracts. The behavior and state of object types are paramount, and language features, such as classes, interfaces, inheritance, and polymorphism, are provided to address these concerns.
In contrast, functional programming approaches computational problems as an exercise in the evaluation of pure functional transformations of data collections. Functional programming avoids state and mutable data, and instead emphasizes the application of functions.
The Java programming language also has the volatile keyword, but it is used for a somewhat different purpose. When applied to a field, the Java volatile guarantees that: (In all versions of Java) There is a global ordering on the reads and writes to a volatile variable. This implies that every thread accessing a volatile field will read its current value before continuing, instead of (potentially) using a cached value. (However, there is no guarantee about the relative ordering of volatile reads and writes with regular reads and writes, meaning that it's generally not a useful threading construct.) (In Java 5 or later) Volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex. Using volatile may be faster than a lock, but it will not work in some situations. The range of situations in which volatile is effective was expanded in Java 5; in particular, double-checked locking now works correctly.
In computer science, the happened-before relation (denoted: \to \;) is a relation between the result of two events, such that if one event should happen before another event, the result must reflect that.
PermGen space PermGen space的全称是Permanent Generation space,是指内存的永久保存区域OutOfMemoryError: PermGen space从表面上看就是内存益出，解决方法也一定是加大内存。说说为什么会内存益出：这一部分用于存放Class和Meta的信息,Class在被 Load的时候被放入PermGen space区域，它和和存放Instance的Heap区域不同,GC(Garbage Collection)不会在主程序运行期对PermGen space进行清理，所以如果你的APP会LOAD很多CLASS的话,就很可能出现PermGen space错误。这种错误常见在web服务器对JSP进行pre compile的时候。 如果你的WEB APP下都用了大量的第三方jar, 其大小 超过了jvm默认的大小(4M)那么就会产生此错误信息了。 解决方法： 手动设置MaxPermSize大小 改正方法：-Xms256m -Xmx256m -XX:MaxNewSize=256m -XX:MaxPermSize=256m 修改TOMCAT_HOME/bin/catalina.sh JAVA_OPTS="-server -XX:PermSize=64M -XX:MaxPermSize=128m 建议：将相同的第三方jar文件移置到tomcat/shared/lib目录下，这样可以达到减少jar 文档重复占用内存的目的。 Sun文档是这样解释的： java.lang.OutOfMemoryError: PermGen space The detail message PermGen space indicates that the permanent generation is full. The permanent generation is the area of the heap where class and method objects are stored. If an application loads a very large number of classes, then the size of the permanent generation might need to be increased using the -XX:MaxPermSize option. Interned java.lang.String objects are also stored in the permanent generation. The java.lang.String class maintains a pool of strings. When the intern method is invoked, the method checks the pool to see if an equal string is already in the pool. If there is, then the intern method returns it; otherwise it adds the string to the pool. In more precise terms, the java.lang.String.intern method is used to obtain the canonical representation of the string; the result is a reference to the same class instance that would be returned if that string appeared as a literal. If an application interns a huge number of strings, the permanent generation might need to be increased from its default setting. When this kind of error occurs, the text String.intern or ClassLoader.defineClass might appear near the top of the stack trace that is printed. The jmap -permgen command prints statistics for the objects in the permanent generation, including information about internalized String instances. See 2.6.4 Getting Information on the Permanent Generation. 下面是某人遇到的问题： SUN JDK+Tomcat 5.5.20运行服务的时候遇到问题，服务器跑几天后就会挂掉，并报java.lang.OutOfMemoryError: PermGen space异常。 发现很多人把问题归因于： spring,hibernate,tomcat，因为他们动态产生类,导致JVM中的permanent heap溢出 。然后解决方法众说纷纭，有人说升级 tomcat版本到最新甚至干脆不用tomcat。还有人怀疑spring的问题，在spring论坛上讨论很激烈，因为spring在AOP时使用CBLIB会动态产生很多类。 但问题是为什么这些王牌的开源会出现同一个问题呢，那么是不是更基础的原因呢？tomcat在Q&A很隐晦的回答了这一点。（Why does the memory usage increase when I redeploy a web application? Because the Classloader (and the Class objects it loaded) cannot be recycled. They are stored in the permanent heap generation by the JVM, and when you redepoy a new class loader is created, which loads another copy of all these classes. This can cause OufOfMemoryErrors eventually. ） 于是有人对更基础的JVM做了检查，发现了问题的关键。原来SUN 的JVM把内存分了不同的区，其中一个就是permanent区用来存放用得非常多的类和类描述。本来SUN设计的时候认为这个区域在JVM启动的时候就固定了，但他没有想到现在动态会用得这么广泛。而且这个区域有特殊的垃圾收回机制，现在的问题是动态加载类到这个区域后，gc根本没办法回收！ 对这个bug最彻底的解决办法就是不要用SUN的JDK，而改用BEA的 JRokit. 在tomcat中redeploy时出现outofmemory的错误. 可以有以下几个方面的原因: １,使用了proxool,因为proxool内部包含了一个老版本的cglib. 2, log4j,最好不用,只用common-logging 3, 老版本的cglib,快点更新到最新版。 ４，更新到最新的hibernate3.2 3、 这里以tomcat环境为例，其它WEB服务器如jboss,weblogic等是同一个道理。 二、java.lang.OutOfMemoryError: Java heap space Heap size 设置 JVM堆的设置是指java程序运行过程中JVM可以调配使用的内存空间的设置.JVM在启动的时候会自动设置Heap size的值， 其初始空间(即-Xms)是物理内存的1/64，最大空间(-Xmx)是物理内存的1/4。可以利用JVM提供的-Xmn -Xms -Xmx等选项可 进行设置。Heap size 的大小是Young Generation 和Tenured Generaion 之和。 提示：在JVM中如果98％的时间是用于GC且可用的Heap size 不足2％的时候将抛出此异常信息。 提示：Heap Size 最大不要超过可用物理内存的80％，一般的要将-Xms和-Xmx选项设置为相同，而-Xmn为1/4的-Xmx值。 解决方法：手动设置Heap size 修改TOMCAT_HOME/bin/catalina.sh 在“echo "Using CATALINA_BASE: $CATALINA_BASE"”上面加入以下行： JAVA_OPTS="-server -Xms800m -Xmx800m -XX:MaxNewSize=256m" 三、实例，以下给出1G内存环境下java jvm 的参数设置参考： JAVA_OPTS="-server -Xms800m -Xmx800m -XX:PermSize=64M -XX:MaxNewSize=256m -XX:MaxPermSize=128m -Djava.awt.headless=true " 针对Tomcat，如果Tomcat下面有多个应用，尽可能的把lib下共用的jar文件放到Tomcat的lib下，发布速度和运行速度上也有所提升。 题外话：经常看到网友抱怨tomcat的性能不如...，不稳定等，其实根据笔者几年的经验，从"互联星空“到现在的房产门户网，我们 均使用tomcat作为WEB服务器，每天访问量百万多，tomcat仍然运行良好。建议大家有问题多从自己程序入手，多看看java的DOC文档。 参考文档：http://blogs.sun.com/jonthecollector/entry/presenting_the_permanent_generation
more on this, http://blogs.sun.com/jonthecollector/
Java handles its memory in two areas. The heap and the stack. We will start with a short overview of memory in general on a computer. Then the Java heap and stack is explained.
2.1. Native Memory
Native memory is the memory which is available to a process, e.g. the Java process. Native memory is controlled by the operating system (OS) and based on physical memory and other physical devices, e.g. disks, flash memory, etc.
The processor (CPU) of the computer computes the instructions to execute and stores its computation results into registers. These registers are fast memory elements which stores the result of the CPU. The processor can access the normal memory over the memory bus. A amount of memory a CPU can access is based on the size of the physical address which the CPU uses to identify physical memory. A 16-bit address can access 2^16 (=65.536) memory locations. A 32-bit address can access 2^32 (=4.294.967.296) memory locations. If each memory area consists of 8 bytes then a 16-bit system can access 64KB of memory and the 32-bit system can access 4GB of memory.
An OS normally uses virtual memory to map the physical memory to memory which each process can see. The OS assigns then memory to each process in a virtual memory space for this process and maps access to this virtual memory to the real physical memory.
Current 32-bit systems uses an extension (Physical Address Extension (PAE)) which extends the physical space to 36-bits of the operation system. This allows the OS to access 64GB. The OS uses then virtual memory to allow the individual process 4 GB of memory. Even with PAE enabled a process can not access more then 4 GB of memory.
Of course with a 64-bit OS this 4GB limitation does not exists any more.
2.2. Memory in Java
Java manages the memory for use. New objects created and placed in the heap. Once your application have no reference anymore to an objects the Java garbage collector is allowed to delete this object and remove the memory so that your application can use this memory again.
2.3. Java Heap
In the heap the Java Virtual Machine (JVM) stores all objects created by the Java application, e.g. by using the “new” operator. The Java garbage collector (gc) can logically separate the heap into different areas, so that the gc can faster identify objects which can get removed
The memory for new objects is allocated on the heap at run time. Instance variables live inside the object in which they are declared.
2.4. Java Stack
Stack is where the method invocations and the the local variables are stored. If a method is called then its stack frame is put onto the top of the call stack. The stack frame holds the state of the method including which line of code is executing and the values of all local variables. The method at the top of the stack is always the current running method for that stack. Threads have their own call stack.
2.5. Escape Analysis
As said earlier in Java objects are created in the heap. The programming language does not offer the possibility to let the programmer decide if an objects should be generated in the stack. But in certain cases it would be desirable to allocate an object on the stack, as the memory allocation on the stack is cheaper then the memory allocation in the heap, deallocation on the stack is free and the stack is efficiently managed by the runtime.
The JVM uses therefore internally escape analysis to check if an object is used only with a thread or method. If the JVM identify this it may decide to create the object on the stack, increasing performance of the Java program.
& from about GC, from Oracle ,http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html
What Is Stack?
Each Java virtual machine thread has a private Java virtual machine stack, created at the same time as the thread. A Java virtual machine stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Because the Java virtual machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated. The memory for a Java virtual machine stack does not need to be contiguous.
The Java virtual machine specification permits Java virtual machine stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the Java virtual machine stacks are of a fixed size, the size of each Java virtual machine stack may be chosen independently when that stack is created. A Java virtual machine implementation may provide the programmer or the user control over the initial size of Java virtual machine stacks, as well as, in the case of dynamically expanding or contracting Java virtual machine stacks, control over the maximum and minimum sizes.
The following exceptional conditions are associated with Java virtual machine stacks:
- If the computation in a thread requires a larger Java virtual machine stack than is permitted, the Java virtual machine throws a StackOverflowError.
- If Java virtual machine stacks can be dynamically expanded, and expansion is attempted but insufficient memory can be made available to effect the expansion, or if insufficient memory can be made available to create the initial Java virtual machine stack for a new thread, the Java virtual machine throws an OutOfMemoryError
What Is Heap?
The Java virtual machine has a heap that is shared among all Java virtual machine threads. The heap is the runtime data area from which memory for all class instances and arrays is allocated.
The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector); objects are never explicitly deallocated. The Java virtual machine assumes no particular type of automatic storage management system, and the storage management technique may be chosen according to the implementor’s system requirements. The heap may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger heap becomes unnecessary. The memory for the heap does not need to be contiguous.
The heap mainly store objects create using or class level variables.
The following exceptional condition is associated with the heap:
- If a computation requires more heap than can be made available by the automatic storage management system, the Java virtual machine throws an OutOfMemoryError