What’s memory leak？

 How does garbage collection work?
The job of the garbage collector is to find objects that are no longer needed by an application and to remove them when they can no longer be accessed or referenced. The garbage collector starts at the root nodes, classes that persist throughout the life of a Java application, and sweeps though all of the nodes that are referenced. As it traverses the nodes, it keeps track of which objects are actively being referenced. Any classes that are no longer being referenced are then eligible to be garbage collected. The memory resources used by these objects can be returned to the Java virtual machine (JVM) when the objects are deleted.
So it is true that Java code does not require the programmer to be responsible for memory management cleanup, and that it automatically garbage collects unused objects. However, the key point to remember is that an object is only counted as being unused when it is no longer referenced. Figure 1 illustrates this concept.

 For Example
The figure illustrates two classes that have different lifetimes during the execution of a Java application. Class A is instantiated first and exists for a long time or for the entire life of the program. At some point, class B is created, and class A adds a reference to this newly created class. Now let's suppose class B is some user interface widget that is displayed and eventually dismissed by the user. Even though class B is no longer needed, if the reference that class A has to class B is not cleared, class B will continue to exist and to take up memory space even after the next garbage collection cycle is executed.

 How the JVM uses system memory for its heap
When running java.exe, you can use certain options to control the startup and maximum size of the garbage-collected heap (-ms and -mx, respectively). The Sun JDK 1.1.8 uses a default 1 MB startup setting and a 16 MB maximum setting. The IBM JDK 1.1.8 uses a default maximum setting of one-half the total physical memory size of the machine. These memory settings have a direct impact on what the JVM does when it runs out of memory. The JVM may continue growing the heap rather than wait for a garbage collection cycle to complete.