使用javap工具分析Java String(字符串)操作

Created by Jerry Wang, last modified on Oct 06, 2016

Put this line into class main method: String a = “i042416”; And decompile the .class file using javap:

We can see the “i042416” is included in the constant pool:

The java code String a = “i042416” is implemented via two lines of codes below:

(1) When the class is loaded by JVM, the string “i042416” is represented by #16. When instruction ldc #16 is called for the first time, the native method StringTable::intern will be called to generate char[], and store the reference into StringTable and constant pool. When the ldc #16 is called subsequently, the reference #16 is directly returned from constant pool.

(2) Instruction astore_1 stores the reference of “i042416” to local variable table. Test via these lines instead: String aa1 = “i042416”; String aa2 = “jerrywang”; String aa3 = “i042416” + “jerrywang”; We can see for line aa3, the string concatenation is done in compilation time.

As a result, the following println gets true as result: String aa1 = “i042416jerrywang”; String aa2 = “jerrywang”; String aa3 = “i042416” + “jerrywang”; System.out.println(aa1 == aa3);