The concept deadlock
"Deadlock" means:
Multiple threads each occupy some shared resources, and other threads wait for each other's resources to be occupied, resulting in two or more threads are waiting for each other to release resources, the situation stopped execution.
Therefore, a need to have a synchronized block when a "lock two or more objects," on issues of "deadlock" may occur. The following cases, "make threads" need to have "mirror object", "Lipstick object" to run sync blocks. So, the actual running time, "Xiaoya makeup threads" have "mirror object", "Big Hell makeup threads" have "lipstick object" are waiting for each other to release resources with each other, in order to make-up. In this way, two threads wait for each other to form a "deadlock state" can not continue to run.
Deadlock demo:
class Lipstick {//口红类
}
class Mirror {//镜子类
}
class Makeup extends Thread {//化妆类继承了Thread类
int flag;
String girl;
static Lipstick lipstick = new Lipstick();
static Mirror mirror = new Mirror();
@Override
public void run() {
// TODO Auto-generated method stub
doMakeup();
}
void doMakeup() {
if (flag == 0) {
synchronized (lipstick) {//需要得到口红的“锁”；
System.out.println(girl + "拿着口红！");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (mirror) {//需要得到镜子的“锁”；
System.out.println(girl + "拿着镜子！");
}
}
} else {
synchronized (mirror) {
System.out.println(girl + "拿着镜子！");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lipstick) {
System.out.println(girl + "拿着口红！");
}
}
}
}
}
public class TestDeadLock {
public static void main(String[] args) {
Makeup m1 = new Makeup();//大丫的化妆线程；
m1.girl = "大丫";
m1.flag = 0;
Makeup m2 = new Makeup();//小丫的化妆线程；
m2.girl = "小丫";
m2.flag = 1;
m1.start();
m2.start();
}
}The results as shown (two threads are waiting for each other's resources are at a standstill):
Deadlock solution
Deadlock was due to "sync blocks need to hold multiple objects locks cause", to solve this problem, the idea is very simple: the same block, do not hold two objects at the same time lock. As described above deadlock case, modified to the example shown.
Deadlock solution to the problem:
class Lipstick {//口红类
}
class Mirror {//镜子类
}
class Makeup extends Thread {//化妆类继承了Thread类
int flag;
String girl;
static Lipstick lipstick = new Lipstick();
static Mirror mirror = new Mirror();
@Override
public void run() {
// TODO Auto-generated method stub
doMakeup();
}
void doMakeup() {
if (flag == 0) {
synchronized (lipstick) {
System.out.println(girl + "拿着口红！");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized (mirror) {
System.out.println(girl + "拿着镜子！");
}
} else {
synchronized (mirror) {
System.out.println(girl + "拿着镜子！");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized (lipstick) {
System.out.println(girl + "拿着口红！");
}
}
}
}
public class TestDeadLock {
public static void main(String[] args) {
Makeup m1 = new Makeup();// 大丫的化妆线程；
m1.girl = "大丫";
m1.flag = 0;
Makeup m2 = new Makeup();// 小丫的化妆线程；
m2.girl = "小丫";
m2.flag = 1;
m1.start();
m2.start();
}
}The results as shown (two threads can get the resources needed to end the program running):
