垃圾回收中的几个概念（未完，待补充）

大对象：（概念取自深入理解java虚拟机 jvm最佳实践）所谓的大对象是指需要大量连续内存空间的java对象，最典型的大对象就是那种很长的字符串以及数组。大对象对虚拟机的内存分配来说是一个坏消息，经常出现的大对象容易导致内存还有不少空间时就提前触发垃圾收集以及获取足够的连续的内存空间来存放

 

 

GC ROOTS

  访问连接Help - Eclipse Platform  中的解释是：

Garbage Collection Roots
A garbage collection root is an object that is accessible from outside the heap. The following reasons make an object a GC root:

System Class
Class loaded by bootstrap/system class loader. For example, everything from the rt.jar like java.util.* .
JNI Local
Local variable in native code, such as user defined JNI code or JVM internal code.
JNI Global
Global variable in native code, such as user defined JNI code or JVM internal code.
Thread Block
Object referred to from a currently active thread block.
Thread
A started, but not stopped, thread.
Busy Monitor
Everything that has called wait() or notify() or that is synchronized. For example, by calling synchronized(Object) or by entering a synchronized method. Static method means class, non-static method means object.
Java Local
Local variable. For example, input parameters or locally created objects of methods that are still in the stack of a thread.
Native Stack
In or out parameters in native code, such as user defined JNI code or JVM internal code. This is often the case as many methods have native parts and the objects handled as method parameters become GC roots. For example, parameters used for file/network I/O methods or reflection.
Finalizable
An object which is in a queue awaiting its finalizer to be run.
Unfinalized
An object which has a finalize method, but has not been finalized and is not yet on the finalizer queue.
Unreachable
An object which is unreachable from any other root, but has been marked as a root by MAT to retain objects which otherwise would not be included in the analysis.
Java Stack Frame
A Java stack frame, holding local variables. Only generated when the dump is parsed with the preference set to treat Java stack frames as objects.
Unknown
An object of unknown root type. Some dumps, such as IBM Portable Heap Dump files, do not have root information. For these dumps the MAT parser marks objects which are have no inbound references or are unreachable from any other root as roots of this type. This ensures that MAT retains all the objects in the dump.

                  常说的GC(Garbage Collector) Roots，特指的是垃圾收集器（Garbage Collector）的对象，GC会收集那些不是GC Roots且没有被GC Roots引用的对象。

      一个对象可以属于多个root，GC Roots有以下几种：

• Class - 由系统类加载器(system class loader)加载的对象，这些类是不能够被回收的，他们可以以静态字段的方式保存持有其它对象。我们需要注意的一点就是，通过用户自定义的类加载器加载的类，除非相应的Java.lang.Class实例以其它的某种（或多种）方式成为roots，否则它们并不是roots。
• Thread - 活着的线程
• Stack Local - Java方法的local变量或参数
• JNI Local - JNI方法的local变量或参数
• JNI Global - 全局JNI引用
• Monitor Used - 用于同步的监控对象
• Held by JVM - 用于JVM特殊目的由GC保留的对象，但实际上这个与JVM的实现是有关的。可能已知的一些类型是：系统类加载器、一些JVM知道的重要的异常类、一些用于处理异常的预分配对象以及一些自定义的类加载器等。然而，JVM并没有为这些对象提供其它的信息，因此需要去确定哪些是属于"JVM持有"的了。

 

           到这里对gc roots 应该基本了解了

对象可达

   对象可达是基于gc roots,通过以一系列的gc roots为起始点，从这些节点向下搜索，搜索所走过的路径称为引用链，一个对象到gc roots 没有任何引用链相连就说这个对象不可达，也就是这个对象空间是可以被gc 回收的

圈起来的是跟gc roots 不可达的

 

 

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在jdk1.9 中G1成了默认的垃圾收集器，替换了原来的cms垃圾收集器。可以说G1的改变很大。以后会详细的补充

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

https://gceasy.io/可以用来分析gc日志