Java synchronized instance member doesn't work in a nested way
3
When programming with Java synchronized, I happend to find a usage which doesn't work as I expect. That is, in thread A, it accesses instance member (content) inside two nested synchronized blocks:
synchronized(this){
synchronized (content) {content = str;}
}
while in thread B, it accesses the same content in only one synchronized block:
synchronized (content) {content = str;}
I expected this can work as a normal usage of
synchronized (content) {content = str;}
however, it doesn't. It works as there is no synchronized.
The complete code and logs are as following:
public class JavaSync {
public static void main(String args) {
SyncContent syncContent = new SyncContent("JavaSync");
ThreadA a = new ThreadA(syncContent);
a.setName("A");
a.start();
ThreadB b = new ThreadB(syncContent);
b.setName("B");
b.start();
ThreadC c = new ThreadC(syncContent);
c.setName("C");
c.start();
}
}
class SyncContent {
volatile String content = new String();
public SyncContent(String content) {
this.content = content;
}
private double timeConsuming() {
double a, b, c;
double sum = 0;
for (int i = 1; i < 2000000; i++) {
a = i + sum / ( i * 19);
b = a / 17;
c = b * 23;
sum += (b + c - a) / (a + i);
}
return sum;
}
synchronized public void syncFunc(String str) {
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
synchronized (content) {
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content new: " + content);
//Thread.sleep(2000); // InterruptedException
System.out.println("syncFunc.Thread: dummy result: " + timeConsuming());
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content final: " + content);
}
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
/*try {
} catch (Exception e) {
e.printStackTrace();
}*/
}
public void syncThis(String str) {
synchronized(this) {
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
synchronized (content) {
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content new: " + content);
//Thread.sleep(2000); // InterruptedException
System.out.println("syncThis.Thread: dummy result: " + timeConsuming());
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content final: " + content);
}
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
/*try {
} catch (Exception e) {
e.printStackTrace();
}*/
}
}
public void syncVariable(String str) {
synchronized(content) {
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " content new: " + content);
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
}
}
}
class ThreadA extends Thread {
private SyncContent syncContent;
private String me = "ThreadA";
public ThreadA(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncThis(me);
}
}
class ThreadB extends Thread {
private SyncContent syncContent;
private String me = "ThreadB";
public ThreadB(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncFunc(me);
}
}
class ThreadC extends Thread {
private SyncContent syncContent;
private String me = "ThreadC";
public ThreadC(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncVariable(me);
}
}
Logs:
syncThis.Thread: A enter: 1542076529822
syncThis.Thread: A content old: JavaSync
syncThis.Thread: A content new: ThreadA
syncVariable.Thread: C enter: 1542076529823
syncVariable.Thread: C content old: ThreadA
syncVariable.Thread: C content new: ThreadC
syncVariable.Thread: C exit: 1542076529824
syncThis.Thread: dummy result: 411764.5149938948
syncThis.Thread: A content final: ThreadC
syncThis.Thread: A exit: 1542076529862
syncFunc.Thread: B enter: 1542076529862
syncFunc.Thread: B content old: ThreadC
syncFunc.Thread: B content new: ThreadB
syncFunc.Thread: dummy result: 411764.5149938948
syncFunc.Thread: B content final: ThreadB
syncFunc.Thread: B exit: 1542076529897
Why doesn't this happen?
java multithreading synchronized
edited Nov 13 '18 at 6:53
Mureinik
180k22130199
asked Nov 13 '18 at 6:30
Michael Wang
183
|
show 1 more comment
3
When programming with Java synchronized, I happend to find a usage which doesn't work as I expect. That is, in thread A, it accesses instance member (content) inside two nested synchronized blocks:
synchronized(this){
synchronized (content) {content = str;}
}
while in thread B, it accesses the same content in only one synchronized block:
synchronized (content) {content = str;}
I expected this can work as a normal usage of
synchronized (content) {content = str;}
however, it doesn't. It works as there is no synchronized.
The complete code and logs are as following:
public class JavaSync {
public static void main(String args) {
SyncContent syncContent = new SyncContent("JavaSync");
ThreadA a = new ThreadA(syncContent);
a.setName("A");
a.start();
ThreadB b = new ThreadB(syncContent);
b.setName("B");
b.start();
ThreadC c = new ThreadC(syncContent);
c.setName("C");
c.start();
}
}
class SyncContent {
volatile String content = new String();
public SyncContent(String content) {
this.content = content;
}
private double timeConsuming() {
double a, b, c;
double sum = 0;
for (int i = 1; i < 2000000; i++) {
a = i + sum / ( i * 19);
b = a / 17;
c = b * 23;
sum += (b + c - a) / (a + i);
}
return sum;
}
synchronized public void syncFunc(String str) {
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
synchronized (content) {
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content new: " + content);
//Thread.sleep(2000); // InterruptedException
System.out.println("syncFunc.Thread: dummy result: " + timeConsuming());
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content final: " + content);
}
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
/*try {
} catch (Exception e) {
e.printStackTrace();
}*/
}
public void syncThis(String str) {
synchronized(this) {
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
synchronized (content) {
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content new: " + content);
//Thread.sleep(2000); // InterruptedException
System.out.println("syncThis.Thread: dummy result: " + timeConsuming());
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content final: " + content);
}
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
/*try {
} catch (Exception e) {
e.printStackTrace();
}*/
}
}
public void syncVariable(String str) {
synchronized(content) {
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " content new: " + content);
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
}
}
}
class ThreadA extends Thread {
private SyncContent syncContent;
private String me = "ThreadA";
public ThreadA(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncThis(me);
}
}
class ThreadB extends Thread {
private SyncContent syncContent;
private String me = "ThreadB";
public ThreadB(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncFunc(me);
}
}
class ThreadC extends Thread {
private SyncContent syncContent;
private String me = "ThreadC";
public ThreadC(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncVariable(me);
}
}
Logs:
syncThis.Thread: A enter: 1542076529822
syncThis.Thread: A content old: JavaSync
syncThis.Thread: A content new: ThreadA
syncVariable.Thread: C enter: 1542076529823
syncVariable.Thread: C content old: ThreadA
syncVariable.Thread: C content new: ThreadC
syncVariable.Thread: C exit: 1542076529824
syncThis.Thread: dummy result: 411764.5149938948
syncThis.Thread: A content final: ThreadC
syncThis.Thread: A exit: 1542076529862
syncFunc.Thread: B enter: 1542076529862
syncFunc.Thread: B content old: ThreadC
syncFunc.Thread: B content new: ThreadB
syncFunc.Thread: dummy result: 411764.5149938948
syncFunc.Thread: B content final: ThreadB
syncFunc.Thread: B exit: 1542076529897
Why doesn't this happen?
java multithreading synchronized
edited Nov 13 '18 at 6:53
Mureinik
180k22130199
asked Nov 13 '18 at 6:30
Michael Wang
183
2
It's not clear what you're asking. Is there a problem? What's the expected result? What's the actual result?
– Charlie
Nov 13 '18 at 6:36
1
considersynchronized (content) { content = str;// Strings are immutatable - what you arelockingon has changed
– Scary Wombat
Nov 13 '18 at 6:40
Probably because none of those locks are common. You are synchronizing on different objects. Each time you create aSynConcentyou create a new string. That won't work.
– markspace
Nov 13 '18 at 6:40
@Charlie, it's expected to synchrolize the instance member content.
– Michael Wang
Nov 13 '18 at 9:43
@ScaryWombat Correct.
– Michael Wang
Nov 13 '18 at 9:45
|
show 1 more comment
3
3
3
When programming with Java synchronized, I happend to find a usage which doesn't work as I expect. That is, in thread A, it accesses instance member (content) inside two nested synchronized blocks:
synchronized(this){
synchronized (content) {content = str;}
}
while in thread B, it accesses the same content in only one synchronized block:
synchronized (content) {content = str;}
I expected this can work as a normal usage of
synchronized (content) {content = str;}
however, it doesn't. It works as there is no synchronized.
The complete code and logs are as following:
public class JavaSync {
public static void main(String args) {
SyncContent syncContent = new SyncContent("JavaSync");
ThreadA a = new ThreadA(syncContent);
a.setName("A");
a.start();
ThreadB b = new ThreadB(syncContent);
b.setName("B");
b.start();
ThreadC c = new ThreadC(syncContent);
c.setName("C");
c.start();
}
}
class SyncContent {
volatile String content = new String();
public SyncContent(String content) {
this.content = content;
}
private double timeConsuming() {
double a, b, c;
double sum = 0;
for (int i = 1; i < 2000000; i++) {
a = i + sum / ( i * 19);
b = a / 17;
c = b * 23;
sum += (b + c - a) / (a + i);
}
return sum;
}
synchronized public void syncFunc(String str) {
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
synchronized (content) {
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content new: " + content);
//Thread.sleep(2000); // InterruptedException
System.out.println("syncFunc.Thread: dummy result: " + timeConsuming());
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content final: " + content);
}
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
/*try {
} catch (Exception e) {
e.printStackTrace();
}*/
}
public void syncThis(String str) {
synchronized(this) {
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
synchronized (content) {
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content new: " + content);
//Thread.sleep(2000); // InterruptedException
System.out.println("syncThis.Thread: dummy result: " + timeConsuming());
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content final: " + content);
}
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
/*try {
} catch (Exception e) {
e.printStackTrace();
}*/
}
}
public void syncVariable(String str) {
synchronized(content) {
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " content new: " + content);
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
}
}
}
class ThreadA extends Thread {
private SyncContent syncContent;
private String me = "ThreadA";
public ThreadA(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncThis(me);
}
}
class ThreadB extends Thread {
private SyncContent syncContent;
private String me = "ThreadB";
public ThreadB(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncFunc(me);
}
}
class ThreadC extends Thread {
private SyncContent syncContent;
private String me = "ThreadC";
public ThreadC(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncVariable(me);
}
}
Logs:
syncThis.Thread: A enter: 1542076529822
syncThis.Thread: A content old: JavaSync
syncThis.Thread: A content new: ThreadA
syncVariable.Thread: C enter: 1542076529823
syncVariable.Thread: C content old: ThreadA
syncVariable.Thread: C content new: ThreadC
syncVariable.Thread: C exit: 1542076529824
syncThis.Thread: dummy result: 411764.5149938948
syncThis.Thread: A content final: ThreadC
syncThis.Thread: A exit: 1542076529862
syncFunc.Thread: B enter: 1542076529862
syncFunc.Thread: B content old: ThreadC
syncFunc.Thread: B content new: ThreadB
syncFunc.Thread: dummy result: 411764.5149938948
syncFunc.Thread: B content final: ThreadB
syncFunc.Thread: B exit: 1542076529897
Why doesn't this happen?
java multithreading synchronized
edited Nov 13 '18 at 6:53
Mureinik
180k22130199
asked Nov 13 '18 at 6:30
Michael Wang
183
When programming with Java synchronized, I happend to find a usage which doesn't work as I expect. That is, in thread A, it accesses instance member (content) inside two nested synchronized blocks:
synchronized(this){
synchronized (content) {content = str;}
}
while in thread B, it accesses the same content in only one synchronized block:
synchronized (content) {content = str;}
I expected this can work as a normal usage of
synchronized (content) {content = str;}
however, it doesn't. It works as there is no synchronized.
The complete code and logs are as following:
public class JavaSync {
public static void main(String args) {
SyncContent syncContent = new SyncContent("JavaSync");
ThreadA a = new ThreadA(syncContent);
a.setName("A");
a.start();
ThreadB b = new ThreadB(syncContent);
b.setName("B");
b.start();
ThreadC c = new ThreadC(syncContent);
c.setName("C");
c.start();
}
}
class SyncContent {
volatile String content = new String();
public SyncContent(String content) {
this.content = content;
}
private double timeConsuming() {
double a, b, c;
double sum = 0;
for (int i = 1; i < 2000000; i++) {
a = i + sum / ( i * 19);
b = a / 17;
c = b * 23;
sum += (b + c - a) / (a + i);
}
return sum;
}
synchronized public void syncFunc(String str) {
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
synchronized (content) {
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content new: " + content);
//Thread.sleep(2000); // InterruptedException
System.out.println("syncFunc.Thread: dummy result: " + timeConsuming());
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " content final: " + content);
}
System.out.println("syncFunc.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
/*try {
} catch (Exception e) {
e.printStackTrace();
}*/
}
public void syncThis(String str) {
synchronized(this) {
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
synchronized (content) {
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content new: " + content);
//Thread.sleep(2000); // InterruptedException
System.out.println("syncThis.Thread: dummy result: " + timeConsuming());
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " content final: " + content);
}
System.out.println("syncThis.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
/*try {
} catch (Exception e) {
e.printStackTrace();
}*/
}
}
public void syncVariable(String str) {
synchronized(content) {
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " enter: " + System.currentTimeMillis());
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " content old: " + content);
content = str;
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " content new: " + content);
System.out.println("syncVariable.Thread: " + Thread.currentThread().getName() + " exit: " + System.currentTimeMillis());
}
}
}
class ThreadA extends Thread {
private SyncContent syncContent;
private String me = "ThreadA";
public ThreadA(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncThis(me);
}
}
class ThreadB extends Thread {
private SyncContent syncContent;
private String me = "ThreadB";
public ThreadB(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncFunc(me);
}
}
class ThreadC extends Thread {
private SyncContent syncContent;
private String me = "ThreadC";
public ThreadC(SyncContent syncContent) {
super();
this.syncContent = syncContent;
}
@Override
public void run() {
syncContent.syncVariable(me);
}
}
Logs:
syncThis.Thread: A enter: 1542076529822
syncThis.Thread: A content old: JavaSync
syncThis.Thread: A content new: ThreadA
syncVariable.Thread: C enter: 1542076529823
syncVariable.Thread: C content old: ThreadA
syncVariable.Thread: C content new: ThreadC
syncVariable.Thread: C exit: 1542076529824
syncThis.Thread: dummy result: 411764.5149938948
syncThis.Thread: A content final: ThreadC
syncThis.Thread: A exit: 1542076529862
syncFunc.Thread: B enter: 1542076529862
syncFunc.Thread: B content old: ThreadC
syncFunc.Thread: B content new: ThreadB
syncFunc.Thread: dummy result: 411764.5149938948
syncFunc.Thread: B content final: ThreadB
syncFunc.Thread: B exit: 1542076529897
Why doesn't this happen?
java multithreading synchronized
java multithreading synchronized
edited Nov 13 '18 at 6:53
Mureinik
180k22130199
asked Nov 13 '18 at 6:30
Michael Wang
183
edited Nov 13 '18 at 6:53
Mureinik
180k22130199
asked Nov 13 '18 at 6:30
Michael Wang
183
edited Nov 13 '18 at 6:53
Mureinik
180k22130199
edited Nov 13 '18 at 6:53
Mureinik
180k22130199
edited Nov 13 '18 at 6:53
Mureinik
180k22130199
180k22130199
asked Nov 13 '18 at 6:30
Michael Wang
183
asked Nov 13 '18 at 6:30
Michael Wang
183
asked Nov 13 '18 at 6:30
Michael Wang
183
183
2
It's not clear what you're asking. Is there a problem? What's the expected result? What's the actual result?
– Charlie
Nov 13 '18 at 6:36
1
considersynchronized (content) { content = str;// Strings are immutatable - what you arelockingon has changed
– Scary Wombat
Nov 13 '18 at 6:40
Probably because none of those locks are common. You are synchronizing on different objects. Each time you create aSynConcentyou create a new string. That won't work.
– markspace
Nov 13 '18 at 6:40
@Charlie, it's expected to synchrolize the instance member content.
– Michael Wang
Nov 13 '18 at 9:43
@ScaryWombat Correct.
– Michael Wang
Nov 13 '18 at 9:45
|
show 1 more comment
2
It's not clear what you're asking. Is there a problem? What's the expected result? What's the actual result?
– Charlie
Nov 13 '18 at 6:36
1
considersynchronized (content) { content = str;// Strings are immutatable - what you arelockingon has changed
– Scary Wombat
Nov 13 '18 at 6:40
Probably because none of those locks are common. You are synchronizing on different objects. Each time you create aSynConcentyou create a new string. That won't work.
– markspace
Nov 13 '18 at 6:40
@Charlie, it's expected to synchrolize the instance member content.
– Michael Wang
Nov 13 '18 at 9:43
@ScaryWombat Correct.
– Michael Wang
Nov 13 '18 at 9:45
2
2
It's not clear what you're asking. Is there a problem? What's the expected result? What's the actual result?
– Charlie
Nov 13 '18 at 6:36
It's not clear what you're asking. Is there a problem? What's the expected result? What's the actual result?
– Charlie
Nov 13 '18 at 6:36
1
1
consider
synchronized (content) { content = str; // Strings are immutatable - what you are locking on has changed– Scary Wombat
Nov 13 '18 at 6:40
consider
synchronized (content) { content = str; // Strings are immutatable - what you are locking on has changed– Scary Wombat
Nov 13 '18 at 6:40
Probably because none of those locks are common. You are synchronizing on different objects. Each time you create a
SynConcent you create a new string. That won't work.– markspace
Nov 13 '18 at 6:40
Probably because none of those locks are common. You are synchronizing on different objects. Each time you create a
SynConcent you create a new string. That won't work.– markspace
Nov 13 '18 at 6:40
@Charlie, it's expected to synchrolize the instance member content.
– Michael Wang
Nov 13 '18 at 9:43
@Charlie, it's expected to synchrolize the instance member content.
– Michael Wang
Nov 13 '18 at 9:43
@ScaryWombat Correct.
– Michael Wang
Nov 13 '18 at 9:45
@ScaryWombat Correct.
– Michael Wang
Nov 13 '18 at 9:45
|
show 1 more comment
1 Answer
1
active
oldest
votes
5
synchronized applies to an object, not a name of a variable. You enter the synchronized block by locking content, but then in the block, you assign a different object to it. The next time you try to synchronize content you'll be synchronizing different object, so the previously held lock won't interfere with it.
If you use a mutable object, such as a StringBuilder and change its data instead, you'll see the behavior you expect:
synchronized (content) {
content.setLength(0); // Remove the old content
content.append(str); // Set the new content
// Rest of the code...
answered Nov 13 '18 at 6:49
Mureinik
180k22130199
That's right. I modified the example code and it works correctly now.
– Michael Wang
Nov 13 '18 at 9:41
This is really a good lesson, as it often uses immutatable variables, e.g. string.
– Michael Wang
Nov 13 '18 at 9:59
2
It might be worth pointing out that this mistake can be avoided by always following a simple rule: If you writesynchronized (foo) { ... }thenfooshould be afinalvariable. If the compiler won't let you do that, then you're either doing something very tricky, or else you're making a mistake. And since "tricky" does not usually mean "good", it's usually best to just follow the rule.
– Solomon Slow
Nov 13 '18 at 14:53
@SolomonSlow Good, a simple but useful rule
– Michael Wang
Nov 14 '18 at 1:32
BTW, another solution is, to add a flag (e.g. an integer) and synchrolize it wherever accessing these immutable variables, as changing these immutable variables to mutable ones might be annoying or impossible sometimes.
– Michael Wang
Nov 15 '18 at 2:05
add a comment |
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5
synchronized applies to an object, not a name of a variable. You enter the synchronized block by locking content, but then in the block, you assign a different object to it. The next time you try to synchronize content you'll be synchronizing different object, so the previously held lock won't interfere with it.
If you use a mutable object, such as a StringBuilder and change its data instead, you'll see the behavior you expect:
synchronized (content) {
content.setLength(0); // Remove the old content
content.append(str); // Set the new content
// Rest of the code...
answered Nov 13 '18 at 6:49
Mureinik
180k22130199
That's right. I modified the example code and it works correctly now.
– Michael Wang
Nov 13 '18 at 9:41
This is really a good lesson, as it often uses immutatable variables, e.g. string.
– Michael Wang
Nov 13 '18 at 9:59
2
It might be worth pointing out that this mistake can be avoided by always following a simple rule: If you writesynchronized (foo) { ... }thenfooshould be afinalvariable. If the compiler won't let you do that, then you're either doing something very tricky, or else you're making a mistake. And since "tricky" does not usually mean "good", it's usually best to just follow the rule.
– Solomon Slow
Nov 13 '18 at 14:53
@SolomonSlow Good, a simple but useful rule
– Michael Wang
Nov 14 '18 at 1:32
BTW, another solution is, to add a flag (e.g. an integer) and synchrolize it wherever accessing these immutable variables, as changing these immutable variables to mutable ones might be annoying or impossible sometimes.
– Michael Wang
Nov 15 '18 at 2:05
add a comment |
5
synchronized applies to an object, not a name of a variable. You enter the synchronized block by locking content, but then in the block, you assign a different object to it. The next time you try to synchronize content you'll be synchronizing different object, so the previously held lock won't interfere with it.
If you use a mutable object, such as a StringBuilder and change its data instead, you'll see the behavior you expect:
synchronized (content) {
content.setLength(0); // Remove the old content
content.append(str); // Set the new content
// Rest of the code...
answered Nov 13 '18 at 6:49
Mureinik
180k22130199
That's right. I modified the example code and it works correctly now.
– Michael Wang
Nov 13 '18 at 9:41
This is really a good lesson, as it often uses immutatable variables, e.g. string.
– Michael Wang
Nov 13 '18 at 9:59
2
It might be worth pointing out that this mistake can be avoided by always following a simple rule: If you writesynchronized (foo) { ... }thenfooshould be afinalvariable. If the compiler won't let you do that, then you're either doing something very tricky, or else you're making a mistake. And since "tricky" does not usually mean "good", it's usually best to just follow the rule.
– Solomon Slow
Nov 13 '18 at 14:53
@SolomonSlow Good, a simple but useful rule
– Michael Wang
Nov 14 '18 at 1:32
BTW, another solution is, to add a flag (e.g. an integer) and synchrolize it wherever accessing these immutable variables, as changing these immutable variables to mutable ones might be annoying or impossible sometimes.
– Michael Wang
Nov 15 '18 at 2:05
add a comment |
5
5
5
synchronized applies to an object, not a name of a variable. You enter the synchronized block by locking content, but then in the block, you assign a different object to it. The next time you try to synchronize content you'll be synchronizing different object, so the previously held lock won't interfere with it.
If you use a mutable object, such as a StringBuilder and change its data instead, you'll see the behavior you expect:
synchronized (content) {
content.setLength(0); // Remove the old content
content.append(str); // Set the new content
// Rest of the code...
answered Nov 13 '18 at 6:49
Mureinik
180k22130199
synchronized applies to an object, not a name of a variable. You enter the synchronized block by locking content, but then in the block, you assign a different object to it. The next time you try to synchronize content you'll be synchronizing different object, so the previously held lock won't interfere with it.
If you use a mutable object, such as a StringBuilder and change its data instead, you'll see the behavior you expect:
synchronized (content) {
content.setLength(0); // Remove the old content
content.append(str); // Set the new content
// Rest of the code...
answered Nov 13 '18 at 6:49
Mureinik
180k22130199
answered Nov 13 '18 at 6:49
Mureinik
180k22130199
answered Nov 13 '18 at 6:49
Mureinik
180k22130199
answered Nov 13 '18 at 6:49
Mureinik
180k22130199
180k22130199
That's right. I modified the example code and it works correctly now.
– Michael Wang
Nov 13 '18 at 9:41
This is really a good lesson, as it often uses immutatable variables, e.g. string.
– Michael Wang
Nov 13 '18 at 9:59
2
It might be worth pointing out that this mistake can be avoided by always following a simple rule: If you writesynchronized (foo) { ... }thenfooshould be afinalvariable. If the compiler won't let you do that, then you're either doing something very tricky, or else you're making a mistake. And since "tricky" does not usually mean "good", it's usually best to just follow the rule.
– Solomon Slow
Nov 13 '18 at 14:53
@SolomonSlow Good, a simple but useful rule
– Michael Wang
Nov 14 '18 at 1:32
BTW, another solution is, to add a flag (e.g. an integer) and synchrolize it wherever accessing these immutable variables, as changing these immutable variables to mutable ones might be annoying or impossible sometimes.
– Michael Wang
Nov 15 '18 at 2:05
add a comment |
That's right. I modified the example code and it works correctly now.
– Michael Wang
Nov 13 '18 at 9:41
This is really a good lesson, as it often uses immutatable variables, e.g. string.
– Michael Wang
Nov 13 '18 at 9:59
2
It might be worth pointing out that this mistake can be avoided by always following a simple rule: If you writesynchronized (foo) { ... }thenfooshould be afinalvariable. If the compiler won't let you do that, then you're either doing something very tricky, or else you're making a mistake. And since "tricky" does not usually mean "good", it's usually best to just follow the rule.
– Solomon Slow
Nov 13 '18 at 14:53
@SolomonSlow Good, a simple but useful rule
– Michael Wang
Nov 14 '18 at 1:32
BTW, another solution is, to add a flag (e.g. an integer) and synchrolize it wherever accessing these immutable variables, as changing these immutable variables to mutable ones might be annoying or impossible sometimes.
– Michael Wang
Nov 15 '18 at 2:05
That's right. I modified the example code and it works correctly now.
– Michael Wang
Nov 13 '18 at 9:41
That's right. I modified the example code and it works correctly now.
– Michael Wang
Nov 13 '18 at 9:41
This is really a good lesson, as it often uses immutatable variables, e.g. string.
– Michael Wang
Nov 13 '18 at 9:59
This is really a good lesson, as it often uses immutatable variables, e.g. string.
– Michael Wang
Nov 13 '18 at 9:59
2
2
It might be worth pointing out that this mistake can be avoided by always following a simple rule: If you write
synchronized (foo) { ... } then foo should be a final variable. If the compiler won't let you do that, then you're either doing something very tricky, or else you're making a mistake. And since "tricky" does not usually mean "good", it's usually best to just follow the rule.– Solomon Slow
Nov 13 '18 at 14:53
It might be worth pointing out that this mistake can be avoided by always following a simple rule: If you write
synchronized (foo) { ... } then foo should be a final variable. If the compiler won't let you do that, then you're either doing something very tricky, or else you're making a mistake. And since "tricky" does not usually mean "good", it's usually best to just follow the rule.– Solomon Slow
Nov 13 '18 at 14:53
@SolomonSlow Good, a simple but useful rule
– Michael Wang
Nov 14 '18 at 1:32
@SolomonSlow Good, a simple but useful rule
– Michael Wang
Nov 14 '18 at 1:32
BTW, another solution is, to add a flag (e.g. an integer) and synchrolize it wherever accessing these immutable variables, as changing these immutable variables to mutable ones might be annoying or impossible sometimes.
– Michael Wang
Nov 15 '18 at 2:05
BTW, another solution is, to add a flag (e.g. an integer) and synchrolize it wherever accessing these immutable variables, as changing these immutable variables to mutable ones might be annoying or impossible sometimes.
– Michael Wang
Nov 15 '18 at 2:05
add a comment |
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2
It's not clear what you're asking. Is there a problem? What's the expected result? What's the actual result?
– Charlie
Nov 13 '18 at 6:36
1
consider
synchronized (content) { content = str;// Strings are immutatable - what you arelockingon has changed– Scary Wombat
Nov 13 '18 at 6:40
Probably because none of those locks are common. You are synchronizing on different objects. Each time you create a
SynConcentyou create a new string. That won't work.– markspace
Nov 13 '18 at 6:40
@Charlie, it's expected to synchrolize the instance member content.
– Michael Wang
Nov 13 '18 at 9:43
@ScaryWombat Correct.
– Michael Wang
Nov 13 '18 at 9:45