Java Reference
In-Depth Information
The figure shows three topics placed on a stack.
Book-1
was placed first,
Book-2
second, and
Book-3
third.
Book
-
3
, which is added last onto the stack, represents the top of the stack.
Book-1
, which is added first onto the stack,
represents the bottom of the stack. Adding an element to a stack is called a
push
operation and removing an element
from a stack is called a
pop
operation. Initially, a stack is empty and the first operation is the push operation. When a
stack is being discarded, it must perform an equal number of push and pop operations so it is empty again.
Each thread in Java is allocated a stack to store its temporary data. A thread stores the state of a method
invocation onto its stack. The state of a Java method comprises the parameters' values, local variables, any
intermediate computed values, and the method's return value, if any. A Java stack consists of stack frames. Each frame
stores the state of one method invocation. A new frame is pushed onto a thread's stack for a method invocation. The
frame is popped from a thread's stack when the method completes.
Suppose a thread starts at the
m1()
method. The
m1()
method calls the
m2()
method, which in turn calls the
m3()
method. Figure
9-5
shows the frames on the stack of a thread when methods
m1()
,
m2()
, and
m3()
are called. Note that
the figures shows the frames when the method
m3()
is called from the method
m2()
, which in turn is called from
the method
m1()
.
Top of stack
Top of stack
Top of stack
Top of stack
Frame of m3( )
Frame of m2( )
Frame for m2( )
Frame for m1( )
Frame of m1( )
Frame of m1( )
m2( ) invoked
m3( ) invoked
m1( ) invoked
Initial state
Top of stack
Top of stack
Top of stack
Frame for m2( )
Frame for m1( )
Frame for m1( )
m3( ) finished
m2( ) finished
m1( ) finished
Figure 9-5.
State of the stack of a thread when methods m1(), m2(), and m3() are called
You can get some pieces of information about the stack of a thread at a specific point in time. Note that the state
of a thread's stack is always changing as the program executes. Therefore, you get a snapshot of the stack of a thread as
it existed at the time you requested it. An object of the
java.lang.StackTraceElement
class represents a stack frame.
You can query four pieces of information about a stack frame: class name, file name, method name, and line number.
To get the stack information, you need to call the
getStackTrace()
method of a
Throwable
object. It returns an array
of
StackTraceElement
objects. The first element of the array represents the top stack frame. The last element of the
array represents the bottom stack frame. When you create an object of the
Throwable
class (or any exception class in
Java), it captures the stack of the thread that is executing.
