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Debugging

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Contents目录

History of Java MT

Sequential & Parallel

Different types of bugs

Debugging skills

Part One

History of Java MT01

JDK 1.0: as a basic and essence, preemptive and cooperative multitasking

History of Java MT

JDK 1.2: abolish stop(), suspend(), resume()

JDK 5.0 & JSR: happens-before, Lock-free(CAS)

JDK 6.0 & 7.0: CyclicBarrier, spin lock, CountDownLatch

Part Two

Sequential & Parallel02

Thread Model

Multi threaded apartment:

There is only one block in each

process, which takes control of

more than one thread

Single threaded apartment:

Each thread is divided into a

block in the process, sharing

data in blocks.Single thread:

One thread in one process,

other process must wait until

the previous ends.

Thread model01

02

03

Sequential & Parallel

Sequential

Parallel

pros cons

Faster only if when there is more CPUs because of higher efficiency and full use of CPU

Easier to learn and has less problem, spend no time on shifting from one thread to the other.

There may occur many problem when visiting public variables or methods.

Some CPUs are left waiting and may cause many waste, which result in worse performance.

Part Three

Different types of bugs03

Every thread in the set are

waiting for other thread’s

signal to get start.

Dead lock Tear apart the thread that

can’t be divided by other

threads.

Atomicity violation The non-deterministic of the

order of the executing order

of threads.

Ordering violation Wrongly visit the sharing

memory without proper

synchronization

Data race

Types of bug

Part Four

Debugging skills04

Cancel all threads in dead lock. Cancel some threads in the dead lock until the lock set free.

Force the process to give up some resource until the dead lock is set free.

Rob other process of their resource and relocate enough of it to the dead lock to set the lock free.

Dead lock

Ordering violation

Add lock to protect the atomicity of the operation, preventing it from

being interrupted by other threads, guarantee the execution order to

some extent.

Add lock & Synchronize

Checking the condition is a common solution to the problem. The

critical part is to be sure that the extra conditional statement works at

your will.

Use condition checking

Data race

• A specific kind of race condition involves

checking for a predicate

• Acting on the predicate, while the state

can change between the time of

check and the time of use.

• Strictly speaking, data race is just a

special type of atomicity violation.

Data race

1. lock before line 2 and unlock after

line 6

2. put lines 6–8 and line 8 each into

one critical region.

3. Two patches that separately fix

the above two atomicity violations

could deadlock with each other

Debug method

Atomicity violation

HXY

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Made by 14302010050