Warning ⚠️: This article takes a long time, to be psychologically prepared
Proof: biased locking, lightweight lock, lock real heavyweight
By the
[java java concurrent threading model] notes the link: https://www.cnblogs.com/yuhangwang/p/11256476.html
this article shows: Whenever java threads created corresponding os pthread_create () will create a thread, using synchronized () is bound to call os pthread_mutex_lock () function
synchronized keyword lock status: no lock, tend to lock, lock lightweight, heavyweight lock
This article from the tend to prove the existence of the lock to start, we all know it will be biased locking thread-safe, but the reality is not necessarily mutually exclusive, biased locking the case of synchronized lock object does not have the resources to compete, does not call os pthread_mutex_lock () function; but first initialized using the lock when does one call pthread_mutex_lock () were biased locking
Conjecture:
biased locking certain real
Confirmation method:
1. Modify pthread_mutex_lock () method, to increase the output current os id Linux source code statement glibc library pthread_mutex_lock.c file;
2.java synchronized keyword lock code used to print the front locking thread id (id this thread will thread into real os Id), and both compared with each other 1 2;
3. If you call os pthread_mutex_lock () os-id with the same java thread-id: Description lock really exist, and only appeared once for the same biased locking
Start confirmation:
:( environment to build here note that linux kernel must correspond gilbc library or the compiler unsuccessful )
Linux: CentOS 7 Gilbc: Gilbc 2.19-official gilbc link http://mirror.hust.edu.cn/gnu/glibc/
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Check the environment:
1, first check your linux there is no java environment:
View is available through java, and two command javac
If not then check yum among them, which can provide JDK installation command:
yum search java | grep -i --color jdk
If not, to jdk8 example:
yum install -y java-1.8.0-openjdk.x86_64 java-1.8.0-openjdk-devel.x86_64
Second,
checking inux environment should correspond to those of gIibc version (which does not correspond causes the compiler to fail)
Third, no check installed gcc, to compile C program, it does not install gcc
yum -y install gcc
Fourth, unpack glibc
tar -zxvf glibc-2.19.tar.gz
Fifth, modify the source code glibc
Review: pthread_mutex_lock () (i.e. when printing call the method) of the process, the threadId added Print OS
{your} /glibc-2.19/nptl/pthread_mutex_lock.c the dir---- the pthread_mutex_lock (): the path
the file pthread_mutex_lock.c the pthread_mutex_lock () function under the line 66 be modified to:
the printf (stderr, "% MSG = LU TID \ n-", to pthread_self);
(stderr: file descriptor, pthread_self: current thread id)
After modifying, all subsequent calls pthread_mutex_lock () function will print out your own thread id
Sixth, the compiler just modify a file:
cd glibc-2.19
After compilation to be stored in file location:
mkdir out
Compilation ( after completion override the system default files, the default file backup in advance, in case of emergency )
cd out ../configure --prefix=/usr --disable-profile --enable-add-ons --with-headers=/usr/include --with-binutils=/usr/bin
carried out:
the make // the make after finished then execute make install
Test run:
java
Only calls pthread_mutex_lock () to print out your own thread id:
Pthread_mutex_lock here represent modify the source code under linux glibc libraries () successfully;
The next step was to verify biased locking in the end is not real existence:
java code:
import java.lang.Thread;
public class ThreadTest {
Object o= new Object();
static {
System.loadLibrary( "TestThreadNative" );
}
public static void main(String[] args) {
//打印出主线程
System.out.println("xxxxxxxxxxxxxxxxxxxxxxxxxxxx");
ThreadTest example4Start = new ThreadTest();
example4Start.start();
}
public void start(){
Thread thread = new Thread(){
@Override
public void run() {
while (true){
try {
sync();
} catch (InterruptedException e) {
}
}
}
};
Thread thread2 = new Thread(){
@Override
public void run() {
while (true){
try {
sync();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
};
thread.setName("t1");
thread2.setName("t2");
thread.start();
}
//.c will print out java threaid corresponding threadid os
public native void tid();
Sync void public () {throws InterruptedException
the synchronized (O) {
// ThreadID Java thread id is given jvm not really corresponding to os thread id
//System.out.println(Thread.currentThread (). getId ()) ;
// Get java thread corresponding to print out the real os thread ID
TID ();
}
}
}
Get real thread id (os thread Id) :( c-file correspondence native void tid top java code ())
#include<pthread.h>
#include<stdio.h>
#include<stdlib.h>
#include "Example4Start.h"
JNIEXPORT void JNICALL Java_Example4Start_tid(JNIEnv *env, jobject c1){
printf("current tid:%lu-----\n",pthread_self());
usleep(700);
}
Then go again generated .class, .h, so then executed (this method
[java java concurrent threading model] notes the link: https://www.cnblogs.com/yuhangwang/p/11256476.html
have corresponding execution method, please refer to)
carried out:
java ThreadTest
display:
Real hammer: Red biased locking prove real, biased locking prove synchronized green after 1.6 has been optimized (os function only called once, do not call back )
Similarly lightweight and heavyweight lock lock can prove this out