深入拆解 Java 虚拟机】Exception异常笔记

￡神魔★判官ぃ 2022-02-04 20:17 220阅读 0赞

### 【深入拆解 Java 虚拟机 】Exception异常笔记 ###

*  try-with-resource语法糖
 *  finally

# try-with-resource语法糖 #

try后对象的close方法都会被运行。

package com.exception.demo;
    public class Foo implements AutoCloseable {
        private final String name;
        public Foo(String name) { this.name = name; }
    
        @Override
        public void close() {
            System.out.println(name);
        }
    
        public static void main(String[] args) {
            try (Foo foo0 = new Foo("Foo0"); // try-with-resources
                 Foo foo1 = new Foo("Foo1");
                 Foo foo2 = new Foo("Foo2")) {
                System.out.println("ssssss");
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("22222");
                throw new RuntimeException("Initial");
            }
        }
    }

start
    end
    Foo2
    Foo1
    Foo0
    Exception in thread "main" java.lang.RuntimeException: Initial
    	at com.exception.demo.Foo.main(Foo.java:22)

# finally #

public class Foo2 {
        private int tryBlock;
        private int catchBlock;
        private int finallyBlock;
        private int methodExit;
    
        public void test() {
            for (int i = 0; i < 100; i++) {
                try {
                    tryBlock = 0;
                    if (i < 50) {
                        continue;
                    } else if (i < 80) {
                        break;
                    } else {
                        return;
                    }
                } catch (Exception e) {
                    catchBlock = 1;
                } finally {
                    finallyBlock = 2;
                }
            }
            methodExit = 3;
        }
    }

对应生成的字节码为

public class com.exception.demo.Foo2 {
      public com.exception.demo.Foo2();
        Code:
           0: aload_0
           1: invokespecial #1                  // Method java/lang/Object."<init>":()V
           4: return
    
      public void test();
        Code:
           0: iconst_0
           1: istore_1
           2: iload_1
           3: bipush        100
           5: if_icmpge     75
           8: aload_0
           9: iconst_0
          10: putfield      #2                  // Field tryBlock:I
          13: iload_1
          14: bipush        50
          16: if_icmpge     27
          19: aload_0
          20: iconst_2
          21: putfield      #3                  // Field finallyBlock:I
          24: goto          69
          27: iload_1
          28: bipush        80
          30: if_icmpge     41
          33: aload_0
          34: iconst_2
          35: putfield      #3                  // Field finallyBlock:I
          38: goto          75
          41: aload_0
          42: iconst_2
          43: putfield      #3                  // Field finallyBlock:I
          46: return
          47: astore_2
          48: aload_0
          49: iconst_1
          50: putfield      #5                  // Field catchBlock:I
          53: aload_0
          54: iconst_2
          55: putfield      #3                  // Field finallyBlock:I
          58: goto          69
          61: astore_3
          62: aload_0
          63: iconst_2
          64: putfield      #3                  // Field finallyBlock:I
          67: aload_3
          68: athrow
          69: iinc          1, 1
          72: goto          2
          75: aload_0
          76: iconst_3
          77: putfield      #6                  // Field methodExit:I
          80: return
        Exception table:
           from    to  target type
               8    19    47   Class java/lang/Exception
              27    33    47   Class java/lang/Exception
               8    19    61   any
              27    33    61   any
              47    53    61   any
              61    62    61   any
    }

finally在try、catch后都会运行一次，如果内部还有if等多个分支，那么还会赋值运行多次。