1. 개요
Cass Loader 란 abstrace class로써 Bytecode를 읽어 들여서 class 객체를 생성하는
역활을 담당한다.
Class Loader가 Class를 Loading하는 시점은 ComplieTime이 아닌 Run Time에 Loading이 된다.
프로그래머 가 SampleTest aaa = new SampleTest(); 라는 코드를 처음 실행하면
JVM은 SampleTest라는 Class를 Class Loader를 통해서 SampleTest.clas의 ByteCode를
최초로 메모리에 Load하게 된다.
2. ClassLoader 기술적 특징
- Hierarchical
Class Loader는 Hierarchical 하게 생성이 가능하고, Parent class laoder에서, child class loader를
갖는것과 같이 Class Loader간의 계층형 구조를 갖게 된다. 최상위 Class Loader는
"bootstrap" Class Loader가 위치 하게 된다.
- Delegate load request
Class loading에는 일정한 규칙이 필요하다. 이는 Class Loader가 계측형의 구조를 가지고
있기 때문에, 어느 시점에 Class 로딩 요청을 받으면 상위 Class Loader가 Loding한 Class가
우선권을 가진다.
- Have visibility constraint
Child Class Loader는 Parent Class Loader의 Clss를 찾을수 있지만, 그 반대의 경우는 불가능하다.
- Cannot unload classes
Class Loader에 의해서 Loading된 Class들은 unload 될수가 없다.
3. Class Loader 위임
-
출처 : http://liebus.tistory.com/30
프로젝트 현재 경로 얻기 ( Getting current working directory ) (0) | 2016.06.15 |
---|---|
자바 객체를 byte[] 로 변환하기 (0) | 2015.08.08 |
[ Java / 자바 ] 시스템 독립적인 라인 구분자 (0) | 2015.04.27 |
[ Java / 자바 ] 현재 날짜 및 시간 구하기 (0) | 2015.04.27 |
[Java] 자바로 만든 구구단 소스 (0) | 2015.04.27 |
출처 : http://docs.oracle.com/javase/7/docs/technotes/tools/share/jstat.html
jstat [ generalOption | outputOptions vmid [interval[s|ms] [count]] ]
generalOption
outputOptions
vmid
[protocol:][//]lvmid[@hostname[:port]/servername]The syntax of the vmid string largely corresponds to the syntax of a URI. The vmid can vary from a simple integer representing a local JVM to a more complex construction specifying a communications protocol, port number, and other implementation-specific values. See Virtual Machine Identifier for details.
interval[s|ms]
count
The jstat tool displays performance statistics for an instrumented HotSpot Java virtual machine (JVM). The target JVM is identified by its virtual machine identifier, or vmid option described below.
NOTE: This utility is unsupported and may not be available in future versions of the JDK. It is not currently available on Windows 98 and Windows ME. platforms.
The syntax of the vmid string largely corresponds to the syntax of a URI:
[protocol:][//]lvmid[@hostname][:port][/servername]
protocol
lvmid
hostname
port
servername
The jstat command supports two types of options, general options and output options. General options cause jstat to display simple usage and version information. Output options determine the content and format of the statistical output.
NOTE: All options, and their functionality are subject to change or removal in future releases.
If you specify one of the general options, you cannot specify any other option or parameter.
If you do not specify a general option, then you can specify output options. Output options determine the content and format of jstat's output, and consist of a single statOption, plus any of the other output options (-h, -t, and -J). The statOption must come first.
Output is formatted as a table, with columns are separated by spaces. A header row with titles describes the columns. Use the -h option to set the frequency at which the header is displayed. Column header names are generally consistent between the different options. In general, if two options provide a column with the same name, then the data source for the two columns are the same.
Use the -t option to display a time stamp column,
labeled Timestamp
as the first column of output. The
Timestamp
column contains the elapsed time, in
seconds, since startup of the target JVM. The resolution of the
time stamp is dependent on various factors and is subject to
variation due to delayed thread scheduling on heavily loaded
systems.
Use the interval and count parameters to determine how frequently and how many times, respectively, jstat displays its output.
NOTE: You are advised not to write scripts to parse jstat's output since the format may change in future releases. If you choose to write scripts that parse jstat output, expect to modify them for future releases of this tool.
Option | Displays... |
---|---|
class | Statistics on the behavior of the class loader. |
compiler | Statistics of the behavior of the HotSpot Just-in-Time compiler. |
gc | Statistics of the behavior of the garbage collected heap. |
gccapacity | Statistics of the capacities of the generations and their corresponding spaces. |
gccause | Summary of garbage collection statistics (same as -gcutil), with the cause of the last and current (if applicable) garbage collection events. |
gcnew | Statistics of the behavior of the new generation. |
gcnewcapacity | Statistics of the sizes of the new generations and its corresponding spaces. |
gcold | Statistics of the behavior of the old and permanent generations. |
gcoldcapacity | Statistics of the sizes of the old generation. |
gcpermcapacity | Statistics of the sizes of the permanent generation. |
gcutil | Summary of garbage collection statistics. |
printcompilation | HotSpot compilation method statistics. |
The following tables summarize the columns that jstat outputs for each statOption.
Column | Description |
---|---|
Loaded | Number of classes loaded. |
Bytes | Number of Kbytes loaded. |
Unloaded | Number of classes unloaded. |
Bytes | Number of Kbytes unloaded. |
Time | Time spent performing class load and unload operations. |
Column | Description |
---|---|
Compiled | Number of compilation tasks performed. |
Failed | Number of compilation tasks that failed. |
Invalid | Number of compilation tasks that were invalidated. |
Time | Time spent performing compilation tasks. |
FailedType | Compile type of the last failed compilation. |
FailedMethod | Class name and method for the last failed compilation. |
Column | Description |
---|---|
S0C | Current survivor space 0 capacity (KB). |
S1C | Current survivor space 1 capacity (KB). |
S0U | Survivor space 0 utilization (KB). |
S1U | Survivor space 1 utilization (KB). |
EC | Current eden space capacity (KB). |
EU | Eden space utilization (KB). |
OC | Current old space capacity (KB). |
OU | Old space utilization (KB). |
PC | Current permanent space capacity (KB). |
PU | Permanent space utilization (KB). |
YGC | Number of young generation GC Events. |
YGCT | Young generation garbage collection time. |
FGC | Number of full GC events. |
FGCT | Full garbage collection time. |
GCT | Total garbage collection time. |
Column | Description |
---|---|
NGCMN | Minimum new generation capacity (KB). |
NGCMX | Maximum new generation capacity (KB). |
NGC | Current new generation capacity (KB). |
S0C | Current survivor space 0 capacity (KB). |
S1C | Current survivor space 1 capacity (KB). |
EC | Current eden space capacity (KB). |
OGCMN | Minimum old generation capacity (KB). |
OGCMX | Maximum old generation capacity (KB). |
OGC | Current old generation capacity (KB). |
OC | Current old space capacity (KB). |
PGCMN | Minimum permanent generation capacity (KB). |
PGCMX | Maximum Permanent generation capacity (KB). |
PGC | Current Permanent generation capacity (KB). |
PC | Current Permanent space capacity (KB). |
YGC | Number of Young generation GC Events. |
FGC | Number of Full GC Events. |
This option displays the same summary of garbage collection statistics as the -gcutil option, but includes the causes of the last garbage collection event and (if applicable) the current garbage collection event. In addition to the columns listed for -gcutil, this option adds the following columns:
Column | Description |
---|---|
LGCC | Cause of last Garbage Collection. |
GCC | Cause of current Garbage Collection. |
Column | Description |
---|---|
S0C | Current survivor space 0 capacity (KB). |
S1C | Current survivor space 1 capacity (KB). |
S0U | Survivor space 0 utilization (KB). |
S1U | Survivor space 1 utilization (KB). |
TT | Tenuring threshold. |
MTT | Maximum tenuring threshold. |
DSS | Desired survivor size (KB). |
EC | Current eden space capacity (KB). |
EU | Eden space utilization (KB). |
YGC | Number of young generation GC events. |
YGCT | Young generation garbage collection time. |
Column | Description |
---|---|
NGCMN | Minimum new generation capacity (KB). |
NGCMX | Maximum new generation capacity (KB). |
NGC | Current new generation capacity (KB). |
S0CMX | Maximum survivor space 0 capacity (KB). |
S0C | Current survivor space 0 capacity (KB). |
S1CMX | Maximum survivor space 1 capacity (KB). |
S1C | Current survivor space 1 capacity (KB). |
ECMX | Maximum eden space capacity (KB). |
EC | Current eden space capacity (KB). |
YGC | Number of young generation GC events. |
FGC | Number of Full GC Events. |
Column | Description |
---|---|
PC | Current permanent space capacity (KB). |
PU | Permanent space utilization (KB). |
OC | Current old space capacity (KB). |
OU | old space utilization (KB). |
YGC | Number of young generation GC events. |
FGC | Number of full GC events. |
FGCT | Full garbage collection time. |
GCT | Total garbage collection time. |
Column | Description |
---|---|
OGCMN | Minimum old generation capacity (KB). |
OGCMX | Maximum old generation capacity (KB). |
OGC | Current old generation capacity (KB). |
OC | Current old space capacity (KB). |
YGC | Number of young generation GC events. |
FGC | Number of full GC events. |
FGCT | Full garbage collection time. |
GCT | Total garbage collection time. |
Column | Description |
---|---|
PGCMN | Minimum permanent generation capacity (KB). |
PGCMX | Maximum permanent generation capacity (KB). |
PGC | Current permanent generation capacity (KB). |
PC | Current permanent space capacity (KB). |
YGC | Number of young generation GC events. |
FGC | Number of full GC events. |
FGCT | Full garbage collection time. |
GCT | Total garbage collection time. |
Column | Description |
---|---|
S0 | Survivor space 0 utilization as a percentage of the space's current capacity. |
S1 | Survivor space 1 utilization as a percentage of the space's current capacity. |
E | Eden space utilization as a percentage of the space's current capacity. |
O | Old space utilization as a percentage of the space's current capacity. |
P | Permanent space utilization as a percentage of the space's current capacity. |
YGC | Number of young generation GC events. |
YGCT | Young generation garbage collection time. |
FGC | Number of full GC events. |
FGCT | Full garbage collection time. |
GCT | Total garbage collection time. |
Column | Description |
---|---|
Compiled | Number of compilation tasks performed by the most recently compiled method. |
Size | Number of bytes of bytecode of the most recently compiled method. |
Type | Compilation type of the most recently compiled method. |
Method | Class name and method name identifying the most recently compiled method. Class name uses "/" instead of "." as namespace separator. Method name is the method within the given class. The format for these two fields is consistent with the HotSpot - XX:+PrintComplation option. |
This section presents some examples of monitoring a local JVM with a lvmid of 21891.
This example attaches to lvmid 21891 and takes 7 samples at 250 millisecond intervals and displays the output as specified by the -gcutil option.
jstat -gcutil 21891 250 7
S0 S1 E O P YGC YGCT FGC FGCT GCT
12.44 0.00 27.20 9.49 96.70 78 0.176 5 0.495 0.672
12.44 0.00 62.16 9.49 96.70 78 0.176 5 0.495 0.672
12.44 0.00 83.97 9.49 96.70 78 0.176 5 0.495 0.672
0.00 7.74 0.00 9.51 96.70 79 0.177 5 0.495 0.673
0.00 7.74 23.37 9.51 96.70 79 0.177 5 0.495 0.673
0.00 7.74 43.82 9.51 96.70 79 0.177 5 0.495 0.673
0.00 7.74 58.11 9.51 96.71 79 0.177 5 0.495 0.673
The output of this example shows that a young generation collection occurred between the 3rd and 4th sample. The collection took 0.001 seconds and promoted objects from the eden space (E) to the old space (O), resulting in an increase of old space utilization from 9.49% to 9.51%. Before the collection, the survivor space was 12.44% utilized, but after this collection it is only 7.74% utilized.
This example attaches to lvmid 21891 and takes samples at 250 millisecond intervals and displays the output as specified by -gcutil option. In addition, it uses the -h3 option to output the column header after every 3 lines of data.
jstat -gcnew -h3 21891 250
S0C S1C S0U S1U TT MTT DSS EC EU YGC YGCT
64.0 64.0 0.0 31.7 31 31 32.0 512.0 178.6 249 0.203
64.0 64.0 0.0 31.7 31 31 32.0 512.0 355.5 249 0.203
64.0 64.0 35.4 0.0 2 31 32.0 512.0 21.9 250 0.204
S0C S1C S0U S1U TT MTT DSS EC EU YGC YGCT
64.0 64.0 35.4 0.0 2 31 32.0 512.0 245.9 250 0.204
64.0 64.0 35.4 0.0 2 31 32.0 512.0 421.1 250 0.204
64.0 64.0 0.0 19.0 31 31 32.0 512.0 84.4 251 0.204
S0C S1C S0U S1U TT MTT DSS EC EU YGC YGCT
64.0 64.0 0.0 19.0 31 31 32.0 512.0 306.7 251 0.204
In addition to showing the repeating header string, this example shows that between the 2nd and 3rd samples, a young GC occurred. Its duration was 0.001 seconds. The collection found enough live data that the survivor space 0 utilization (S0U) would would have exceeded the desired survivor Size (DSS). As a result, objects were promoted to the old generation (not visible in this output), and the tenuring threshold (TT) was lowered from 31 to 2.
Another collection occurs between the 5th and 6th samples. This collection found very few survivors and returned the tenuring threshold to 31.
This example attaches to lvmid 21891 and takes 3 samples at 250 millisecond intervals. The -t option is used to generate a time stamp for each sample in the first column.
jstat -gcoldcapacity -t 21891 250 3
Timestamp OGCMN OGCMX OGC OC YGC FGC FGCT GCT
150.1 1408.0 60544.0 11696.0 11696.0 194 80 2.874 3.799
150.4 1408.0 60544.0 13820.0 13820.0 194 81 2.938 3.863
150.7 1408.0 60544.0 13820.0 13820.0 194 81 2.938 3.863
The Timestamp
column reports the elapsed time in
seconds since the start of the target JVM. In addition, the
-gcoldcapacity output shows the old generation capacity
(OGC) and the old space capacity (OC) increasing as the heap
expands to meet allocation and/or promotion demands. The old
generation capacity (OGC) has grown to from 11696 KB to 13820 KB
after the 81st Full GC (FGC). The maximum capacity of the
generation (and space) is 60544 KB (OGCMX), so it still has room
to expand.
This example attaches to lvmid 40496 on the system named remote.domain using the -gcutil option, with samples taken every second indefinitely.
jstat -gcutil 40496@remote.domain 1000
... output omitted
The lvmid is combined with the name of the remote host to construct a vmid of 40496@remote.domain. This vmid results in the use of the rmi protocol to communicate to the default jstatd server on the remote host. The jstatd server is located using the rmiregistry on remote.domain that is bound to the default rmiregistry port (port 1099).
[오라클/oracle] 인덱스를 8192로 USERS 테이블스페이스에서 확장할 수 없습니다 (0) | 2015.04.20 |
---|---|
[오라클/oracle] 파티션된 테이블과 인덱스 정보 조회 (0) | 2015.04.20 |
[오라클/oracle] 테이블 파티션 ( oracle table partitioning ) (0) | 2015.04.20 |
[오라클/oracle] 오라클 DB 에디션 업그레이드 (0) | 2015.04.20 |
[오라클/oracle] 버전 및 에디션 정보 확인 방법 (0) | 2015.04.20 |