June 10, 2018 6 min to read

Java Code Optimization

Java 代码优化

Java代码优化

• 应用层
  • 分清使用场合: 最常见的String与StringBuilder，还有很多。
    • 避免在循环条件中使用复杂表达式

        // Bad
        import java.util.Vector;
        class CEL {
            void method (Vector vector) {
                for (int i = 0; i < vector.size (); i++)  // Violation
                    ; // ...
            }
        }
      
        // Nice
        class CEL_fixed {
            void method (Vector vector) {
                int size = vector.size ()
                for (int i = 0; i < size; i++)
                    ; // ...
            }
        }
      

    • 在finally块中关闭Stream

        // Bad
        import java.io.*;
        public class CS {
            public static void main (String args[]) {
                CS cs = new CS ();
                cs.method ();
            }
            public void method () {
                try {
                    FileInputStream fis = new FileInputStream ("CS.java");
                    int count = 0;
                    while (fis.read () != -1)
                        count++;
                    System.out.println (count);
                    fis.close ();
                } catch (FileNotFoundException e1) {
                } catch (IOException e2) {
                }
            }
        }
      
        // Nice
        import java.io.*;
        public class CS {
            public static void main (String args[]) {
                CS cs = new CS ();
                cs.method ();
            }
            public void method () {
                try {
                    FileInputStream fis = new FileInputStream ("CS.java");
                    int count = 0;
                    while (fis.read () != -1)
                        count++;
                    System.out.println (count);
                } catch (FileNotFoundException e1) {
                } catch (IOException e2) {
                } finally {
                    fis.close ();
                }
            }
        }
      

    • 为’Vectors’ 和 ‘Hashtables’定义初始大小

        // Bad
        import java.util.Vector;
        public class DIC {
            public void addObjects (Object[] o) {
                // if length > 10, Vector needs to expand 
                for (int i = 0; i< o.length;i++) {    
                    v.add(o);   // capacity before it can add more elements.
                }
            }
            public Vector v = new Vector();  // no initialCapacity.
        }
      
        // Nice
        public Vector v = new Vector(20);  
        public Hashtable hash = new Hashtable(10); 
      

    • 使用’System.arraycopy ()’代替通过循环复制数组

        // Bad
        public class IRB
        {
            void method () {
                int[] array1 = new int [100];
                for (int i = 0; i < array1.length; i++) {
                    array1 [i] = i;
                }
                int[] array2 = new int [100];
                for (int i = 0; i < array2.length; i++) {
                    array2 [i] = array1 [i];                 // Violation
                }
            }
        }
      
        // Nice
        public class IRB
        {
            void method () {
                int[] array1 = new int [100];
                for (int i = 0; i < array1.length; i++) {
                    array1 [i] = i;
                }
                int[] array2 = new int [100];
                System.arraycopy(array1, 0, array2, 0, 100);
            }
        }
      

    • 让访问实例变量的getter/setter方法变成”final”

        // Bad
        public class UISO {
            public UISO () {}
        }
        class Dog extends UISO {
            void method (Dog dog, UISO u) {
                Dog d = dog;
                if (d instanceof UISO) // always true.
                    System.out.println("Dog is a UISO"); 
                UISO uiso = u;
                if (uiso instanceof Object) // always true.
                    System.out.println("uiso is an Object"); 
            }
        }
      
        // Nice
        class Dog extends UISO {
            void method () {
                Dog d;
                System.out.println ("Dog is an UISO");
                System.out.println ("UISO is an UISO");
            }
        }
      

    • … …
  • HotSpot: 关键函数调优
  • … …

• JVM 层

  • GC: 垃圾回收已经不再是经常性的性能杀手了。大多数代码的性能关键并不在这一块，此处提出仅是记录。有兴趣的可以了解检测GC情况的方法及如何调优。
  • JIT: 除非自主添加的代码过于复杂，否则也不用考虑这一块。有兴趣的了解JIT机制及使用场景。
  • … …