Two basic approaches to distinguishing live objects from garbage are reference counting and tracing. Reference counting garbage collectors distinguish live objects from garbage objects by keeping a count for each object on the heap. The count keeps track of the number of references to that object. Tracing garbage collectors actually trace out the graph of references starting with the root nodes. Objects that are encountered during the trace are marked in some way. After the trace is complete, unmarked objects are known to be unreachable and can be garbage collected.
1. Reference Counting Collectors:
Reference counting was an early garbage collection strategy. In this approach, a reference count is maintained for each object on the heap. When an object is first created and a reference to it is assigned to a variable, the object's reference count is set to one. When any other variable is assigned a reference to that object, the object's count is incremented. When a reference to an object goes out of scope or is assigned a new value, the object's count is decremented. Any object with a reference count of zero can be garbage collected. When an object is garbage collected, any objects that it refers to have their reference counts decremented. In this way the garbage collection of one object may lead to the subsequent garbage collection of other objects.
An advantage of this approach is that a reference counting collector can run in small chunks of time closely interwoven with the execution of the program. This characteristic makes it particularly suitable for real-time environments where the program can't be interrupted for very long. A disadvantage is that reference counting does not detect cycles: two or more objects that refer to one another. An example of a cycle is a parent object that has a reference to a child object that has a reference back to the parent. These objects will never have a reference count of zero even though they may be unreachable by the roots of the executing program. Another disadvantage of reference counting is the overhead of incrementing and decrementing the reference count each time.
Because of the disadvantages inherent in the reference counting approach, this technique is currently out of favor. It is more likely that the Java virtual machines you encounter in the real world will use a tracing algorithm in their garbage-collected heaps.
2. Tracing Collectors:
Tracing garbage collectors trace out the graph of object references starting with the root nodes. Objects that are encountered during the trace are marked in some way. Marking is generally done by either setting flags in the objects themselves or by setting flags in a separate bitmap. After the trace is complete, unmarked objects are known to be unreachable and can be garbage collected.
The basic tracing algorithm is called "mark and sweep." This name refers to the two phases of the garbage collection process. In the mark phase, the garbage collector traverses the tree of references and marks each object it encounters. In the sweep phase, unmarked objects are freed, and the resulting memory is made available to the executing program. In the Java virtual machine, the sweep phase must include finalization of objects.
1. Reference Counting Collectors:
Reference counting was an early garbage collection strategy. In this approach, a reference count is maintained for each object on the heap. When an object is first created and a reference to it is assigned to a variable, the object's reference count is set to one. When any other variable is assigned a reference to that object, the object's count is incremented. When a reference to an object goes out of scope or is assigned a new value, the object's count is decremented. Any object with a reference count of zero can be garbage collected. When an object is garbage collected, any objects that it refers to have their reference counts decremented. In this way the garbage collection of one object may lead to the subsequent garbage collection of other objects.
An advantage of this approach is that a reference counting collector can run in small chunks of time closely interwoven with the execution of the program. This characteristic makes it particularly suitable for real-time environments where the program can't be interrupted for very long. A disadvantage is that reference counting does not detect cycles: two or more objects that refer to one another. An example of a cycle is a parent object that has a reference to a child object that has a reference back to the parent. These objects will never have a reference count of zero even though they may be unreachable by the roots of the executing program. Another disadvantage of reference counting is the overhead of incrementing and decrementing the reference count each time.
Because of the disadvantages inherent in the reference counting approach, this technique is currently out of favor. It is more likely that the Java virtual machines you encounter in the real world will use a tracing algorithm in their garbage-collected heaps.
2. Tracing Collectors:
Tracing garbage collectors trace out the graph of object references starting with the root nodes. Objects that are encountered during the trace are marked in some way. Marking is generally done by either setting flags in the objects themselves or by setting flags in a separate bitmap. After the trace is complete, unmarked objects are known to be unreachable and can be garbage collected.
The basic tracing algorithm is called "mark and sweep." This name refers to the two phases of the garbage collection process. In the mark phase, the garbage collector traverses the tree of references and marks each object it encounters. In the sweep phase, unmarked objects are freed, and the resulting memory is made available to the executing program. In the Java virtual machine, the sweep phase must include finalization of objects.
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