1、从字符串拼接学习字节码分析

public class JvmTest {
    public void test() {
        String s4 = new String("hel") + "lo";
    }
}

先反汇编，到class文件所在目录打开控制台执行javap -c 类名

$ javap -c JvmTest
警告: 二进制文件JvmTest包含com.chen.base.jvm.JvmTest
Compiled from "JvmTest.java"
public class com.chen.base.jvm.JvmTest {
  public com.chen.base.jvm.JvmTest();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public void test();
    Code:
       0: new           #2                  // class java/lang/StringBuilder
       3: dup
       4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
       7: new           #4                  // class java/lang/String
      10: dup
      11: ldc           #5                  // String hel
      13: invokespecial #6                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
      16: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      19: ldc           #8                  // String lo
      21: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      27: astore_1
      28: return
}

接下来一个指令一个指令的分析，为什么new String("hel") + "lo";创建了两个对象。